docs: describe parse usage

Author: marionebl

@commitlint/parse/README.md

@@ -0,0 +1,29 @@
+> Parse commit messages to structured data
+
+# @commitlint/parse
+
+## Install
+
+```
+npm install --save @commitlint/parse
+```
+
+## Use
+
+```js
+const parse = require('@commitlint/parse');
+```
+
+## API
+
+### parse(message: string, parser: Function, parserOpts: Object)
+
+* **message**: Commit message to parser
+* **parser**: Sync parser function to use. Defaults to `sync` of `conventional-commits-parser`
+* **parserOpts**: Options to pass to `parser`
+  ```js
+  {
+    commentChar: null, // character indicating comment lines
+    issuePrefixes: ['#'] // prefix characters for issue references
+  }
+  ```