Add explainer for LocalDateTime

I intend to eventually adapt this into the documentation for
LocalDateTime, but given the draft status of the API this seems more
appropriate.

docs/localdatetime-draft.md

@@ -0,0 +1,101 @@
+# Zoned Date/Time Type
+
+**Name:** It's currently being referred to as `Temporal.LocalDateTime`, but we already know this is not a good name for reasons discussed years ago.
+This name is not intended to stick.
+
+**What it is:** The LocalDateTime type is intended to refer to an event, that happened (or will happen) in a particular place.
+
+**Motivation:**
+- "Event that happened in a particular place" is a very common use case, perhaps the most common.
+  So we want a type that does the right thing for that use case.
+- Both Absolute and DateTime have cases where they do the right thing for this use case, and cases where they do the wrong thing.
+  (ex.: "Move a meeting three days later" is correct with DateTime and wrong with Absolute; "Postpone a reminder by one hour" is correct with Absolute and wrong with DateTime.)
+- The cases where Absolute and DateTime do the wrong thing will only produce the wrong result when used across a DST transition, so in many places the bugs will only pop up twice per year, if at all.
+- When writing the cookbook, to deal with this use case, we found that it was actually necessary a lot of the time to pass around a record consisting of `{ absolute, timeZone }`.
+  If that is so common, then we may as well have a type for it.
+- Even worse, there is a temptation to pass around a serialized string instead of such a record, which is wrong because the time zone offset may have changed when you read in the string again with `Temporal.Absolute.from()`.
+
+**API**: The API looks something like this:
+
+```typescript
+class Temporal.LocalDateTime {
+  // creation
+  constructor(absolute: Temporal.Absolute, timeZone: Temporal.TimeZone, calendar: Temporal.Calendar);
+  static from(item: string | object, options?: object) : Temporal.LocalDateTime;
+
+  // immutable 'mutation'
+  with(localDateTimeLike: object, options?: object) : Temporal.LocalDateTime;
+  withCalendar(calendar: Temporal.Calendar) : Temporal.LocalDateTime;
+  get startOfDay() : Temporal.LocalDateTime;
+
+  // getters / field access
+  get era(): string | undefined;
+  get year(): number;
+  get month(): number;
+  get day(): number;
+  get hour(): number;
+  get minute(): number;
+  get second(): number;
+  get millisecond(): number;
+  get microsecond(): number;
+  get nanosecond(): number;
+  get timeZone(): Temporal.TimeZone;
+  get timeZoneOffsetNanoseconds(): number;
+  get timeZoneOffsetString(): string;
+  get calendar(): Temporal.Calendar;
+  get dayOfWeek(): number;
+  get dayOfYear(): number;
+  get weekOfYear(): number;
+  get hoursInDay(): number;
+  get daysInMonth(): number;
+  get daysInYear(): number;
+  get isLeapYear(): boolean;
+  get isTimeZoneOffsetTransition(): boolean;
+  getFields(): object;
+  getISOCalendarFields(): object;
+
+  // type conversion
+  toAbsolute(): Temporal.Absolute;
+  toDateTime(): Temporal.DateTime;
+  toDate(): Temporal.Date;
+  toYearMonth(): Temporal.YearMonth;
+  toMonthDay(): Temporal.MonthDay;
+  toTime(): Temporal.Time;
+
+  // comparison
+  static compare(one: Temporal.LocalDateTime, two: Temporal.LocalDateTime): number;
+  equals(other: Temporal.LocalDateTime): boolean;
+
+  // arithmetic
+  plus(duration: Temporal.Duration | object, options?: object) : Temporal.LocalDateTime;
+  minus(duration: Temporal.Duration | object, options?: object) : Temporal.LocalDateTime;
+  difference(other: Temporal.LocalDateTime, options?: object) : Temporal.Duration;
+
+  // serialization / presentation
+  toString() : string;
+  toJSON() : string;
+  toLocaleString(locales?: string | Array<string>, options?: object): string;
+}
+```
+
+(This is not a fully functional description, it's made concise for readability.
+For example, the details of the options bags are omitted.)
+
+It basically has all the API of DateTime, but with a time zone as well, adding the time zone offset for disambiguating near transitions, and several time-zone-related convenience properties.
+But since it represents an unambiguous moment in time (like Absolute, and unlike DateTime), the internal model is more like Absolute with a time zone and calendar.
+
+To create one, you use `Temporal.LocalDateTime.from`, or the `toLocalDateTime()` method of another Temporal type.
+- From a string: `from(string)`
+- From raw DateTime fields: `from({ year, month, day, etc., timeZone, timeZoneOffsetNanoseconds })`
+- From an Absolute: `absolute.toLocalDateTime(timeZone)`
+- From a DateTime, which potentially needs disambiguation: `dateTime.toLocalDateTime(timeZone, { disambiguation })`
+
+As in the other Temporal types, the constructor is more low-level and takes an Absolute, a TimeZone, and a Calendar.
+
+"New" API that is not on DateTime:
+- `hoursInDay` - this is not possible with Absolute (no concept of days and no time zone) nor with DateTime (no time zone), but it makes sense for this type.
+- `timeZoneOffsetNanoseconds` - without this, you'd need to do `ldt.timeZone.getOffsetNanoseconds(ldt.toAbsolute())`.
+- `timeZoneOffsetString` - ditto, you'd need `ldt.timeZone.getOffsetString(ldt.toAbsolute())`.
+- `startOfDay` - this is a convenient way to get the start of the day, especially if the day does not start at 00:00 due to DST.
+  Also not needed with Absolute (no concept of days) nor with DateTime (day always starts at 00:00), but it makes sense for this type.
+- `isTimeZoneOffsetTransition` - convenience property getter.