Create README.md with documentation and usage (#4457)

Per prettier/prettier#2772 (comment)

packages/jest-docblock/README.md

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+# jest-docblock
+
+`jest-docblock` is a package that can extract and parse a specially-formatted comment called a "docblock" at the top of a file.
+
+A docblock looks like this:
+```js
+/**
+ * Stuff goes here!
+ */
+```
+
+Docblocks can contain pragmas, which are words prefixed by `@`:
+```js
+/**
+ * Pragma incoming!
+ *
+ * @flow
+ */
+```
+
+Pragmas can also take arguments:
+```js
+/**
+ * Check this out:
+ *
+ * @myPragma it is so cool
+ */
+```
+
+`jest-docblock` can:
+* extract the docblock from some code as a string
+* parse a docblock string's pragmas into an object
+
+## Installation
+```sh
+# with yarn
+$ yarn add jest-docblock
+# with npm
+$ npm install jest-docblock
+```
+
+## Usage
+
+```js
+const code = `
+/**
+ * Everything is awesome!
+ *
+ * @everything is:awesome
+ * @flow
+ */
+ 
+ export const everything = Object.create(null);
+ export default function isAwesome(something) {
+   return something === everything;
+ }
+`;
+
+const { extract, parse } = require("jest-docblock");
+
+const docblock = extract(code);
+console.log(docblock); // "/**\n * Everything is awesome!\n * \n * @everything is:awesome\n * @flow\n */"
+
+const pragmas = parse(docblock);
+console.log(pragmas); // { everything: "is:awesome", flow: "" }
+```
+
+## API Documentation
+
+### `extract(contents: string): string`
+Extracts a docblock from some file contents. Returns the docblock contained in `contents`. If `contents` did not contain a docblock, it will return the empty string (`""`).
+
+### `parse(docblock: string): {[key: string]: string}`
+Parses the pragmas in a docblock string into an object whose keys are the pragma tags and whose values are the arguments to those pragmas.