Adds some docs for how a formatter works

Author: Orta Therox

src/services/formatting/README.md

@@ -0,0 +1,43 @@
+# How does TypeScript formatting work?
+
+To format code you need to have a formatting context and a sourcefile.  The formatting context contains 
+all user settings like tab size, newline character, etc. The sourcefile is self explanatory. 
+
+The end result of formatting is represented by TextChange objects which hold the new string content, and 
+the text to replace it with. 
+
+```ts
+export interface TextChange {
+    span: TextSpan; // start, length
+    newText: string;
+}
+```
+
+## Internals
+
+Most of the exposed APIs internally are `format*` and they all set up and configure `formatSpan` which could be considered the root call for formatting. Span in this case refers to the range of 
+the sourcefile which should be formatted. 
+
+The formatSpan then uses a scanner (either with or without JSX support) which starts at the highest
+node the covers the span of text and recurses down through the node's children.
+
+As it recurses, `processNode` is called on the children setting the indentation is decided and passed 
+through into each of that node's children.
+
+The meat of formatting decisions is made via `processPair`, the pair here being the current node and the previous node. `processPair` which mutates the formatting context to represent the current place in the scanner and requests a set of rules which can be applied to the items via `createRulesMap`.
+
+There are a lot of rules, which you can find in [rules.ts](./rules.ts) each one has a left and right reference to nodes or token ranges and note of what action should be applied by the formatter.
+
+### Where is this used?
+
+The formatter is used mainly from the language service for formatting with an editor, but [services/textChanges.ts](/src/services/textChanges.ts) also uses this formatter when emitting code for quick fixes. 
+
+The formatter is not exported publicly, and so all usage comes through the language server.
+
+### Sample code
+
+```ts
+const nonFormattedText = `[js source code]`;
+const sourceFile = createSourceFile("any file name", nonFormattedText, ScriptTarget.ESNext, /*setParentNodes*/ true, ScriptKind.JS);
+const changes = formatting.formatDocument(sourceFile, formatContext);
+```