docs(contributing): Add contribution docs

Also add jsdocs on existing methods

Author: Kent C. Dodds

CONTRIBUTING.md

@@ -33,6 +33,10 @@ that I'll accept it before you spend time working on it.
 
 ## Set up instructions
 
+First of all, this is a JavaScript project that's distributed on [npmjs.org](https://npmjs.org) and
+therefore uses JavaScript tooling based on [Node.js](https://nodejs.org/) with dependencies from npm.
+You're going to need to have those things installed to contribute to this project.
+
 1. Fork the repo
 2. Clone your fork
 3. Create a branch

src/flatten.js

@@ -1,5 +1,13 @@
 export default flatten
 
+/**
+ * Original Source: http://stackoverflow.com/a/15030117/971592
+ *
+ * This method will flatten an array given to it.
+ *
+ * @param {Array} arr - The array to flatten
+ * @return {Array} - The flattened array
+ */
 function flatten(arr) {
   return arr.reduce(function flattenReducer(flat, toFlatten) {
     return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten)

src/snake-to-camel.js

@@ -2,6 +2,14 @@ export default snakeToCamel
 
 const regex = /(\-\w)/g
 
+/**
+ * Original Source: http://stackoverflow.com/a/6661012/971592
+ *
+ * This method converts a snake-case string to a camelCase string
+ *
+ * @param {String} s - The snake-case string to camelCase
+ * @return {String} - The camelCase version of the snake-case string
+ */
 function snakeToCamel(s) {
   return s.replace(regex, function snakeToCamelReplacer(m) {
     return m[1].toUpperCase()