docs (no-constructor): add docs (#110)

These are migrated from the custom elements plugin as-is.

docs/rules/no-constructor.md

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+# Disallows constructors in custom element classes (no-constructor)
+
+Classes can use the `constructor` method to execute behaviours upon creation of
+their object. This is useful for things like initialisation of private state.
+Custom Elements also have other lifecycle callbacks which can be used for this,
+such as `connectedCallback`.
+
+For Custom Elements, the `constructor` method can be a little unwieldy to use;
+it is called as soon as the class is instantiated with `document.createElement`
+which will be _before_ the element has been appended to the DOM. It can be
+simpler to put code within the `connectedCallback` which is fired when the
+element has been appended into the DOM.
+
+Many expectations from an element are not true during the `constructor` call.
+For example you cannot:
+
+- read any attributes on the element (`.attributes` will always be an
+empty `NamedNodeMap`).
+- access any child elements (`.children` will always be an empty
+`HTMLCollection`).
+- Fire events that bubble. As the node is not connected it cannot bubble.
+
+Also, there are many edge cases that can cause complications inside the
+constructor, for example:
+
+- Creating child elements and appending them will cause _their_ connected
+callbacks to fire, but their parent (the current element) will be disconnected
+from the DOM.
+- Changing attributes will cause `attributeChangedCallback` to fire, which may
+expect to be connected.
+
+## Rule Details
+
+This rule disallows using the `constructor` in an HTMLElement class.
+
+👎 Examples of **incorrect** code for this rule:
+
+```js
+class FooBar extends HTMLElement {
+  constructor() {
+    super()
+    this.initialState = {}
+  }
+}
+```
+
+👍 Examples of **correct** code for this rule:
+
+```js
+class FooBar extends HTMLElement {
+  connectedCallback() {
+    this.initialState = {}
+  }
+}
+```
+
+## When Not To Use It
+
+If you are comfortable with the trade-offs of using the `constructor` function.