Add contributors documentation

[nojaf]

We should have documentation that explains the high-level overview of this project.
And add some detailed information about the code. The high-level stuff is somewhat captured in our Contribution guidelines. It should be split up and expanded upon.

High-level topics

Somewhat in order.

• A first local build
• Project structure (what does each project do)
• Overview of FAKE build
• Parsing of the AST
• Trivia and ASTTransformer
• CodePrinter
• WriteEvents in Context
• Format selection
• Fantomas Daemon and Fantomas.Client
• Formatting with conditional compiler defines
• Online tools and how to use them.
• PR checklist.

Details

These are in no particular order.

• Explain Context -> Context
• Custom operators: !-, +>
• Helper functions: col, expressionFitsOnRestOfLine, isShortExpression, sepNln, sepSpace, onlyIf, genTriviaFor, indent, atCurrentColumn, colWithNlnWhenItemIsMultiline, leadingExpressionIsMultiline.
• Explain Queue data structure
• Why indent only works after sepNln
• What ColMultilineItem is.
• The Define module and how we resolve the different AST trees.
• AstExtensions and .FullRange extension function.

Questions that should be answered

Again no particular order.

• Why do we have our own F# compiler API?
• Why all the active patterns in SourceParser?
• Why we restore strings/chars/numbers using ISourceText.GetContentAt?
• Why could a code comment disappear after formatting?
• Why is CodePrinter so large and full of recursive functions?