Add contributor guidelines mentioning code-linting

CONTRIBUTING.md

@@ -0,0 +1,47 @@
+
+# Contributing to gmail.js
+
+## Documentation
+
+If you introduce new API calls or features, please ensure they are
+documented. Having it both in the `README.md`-file and as code-doc
+makes it easier to use for everyone!
+
+## Code quality
+
+Gmail.js tries to keep a consistent code-quality and style-guide.
+
+To enforce this, `eslint` is used.
+
+Before submitting any patches, please run check that you have not
+increased the amount of warnings, or caused any linting-errors. You
+can do this by running `npm run lint`:
+
+````bash
+$ npm install
+$ npm run lint
+
+> [email protected] test /home/jostein/build/gmail.js
+> eslint src/*.js
+
+
+/home/jostein/build/gmail.js/src/gmail.js
+   650:66  warning  'j' is defined but never used          no-unused-vars
+   691:48  warning  'xhr' is defined but never used        no-unused-vars
+   942:85  warning  'password' is defined but never used   no-unused-vars
+  1086:44  warning  'response' is defined but never used   no-unused-vars
+  1122:65  warning  'callbacks' is defined but never used  no-unused-vars
+  1442:86  warning  'sub' is defined but never used        no-unused-vars
+
+✖ 6 problems (0 errors, 6 warnings)
+
+````
+
+Submissions with actual linting-errors will not be accepted, and
+linting-warnings should be taken as that: Your code may not be doing
+what you think it does.
+
+## Anything else?
+
+Apart from that: Feel free to send in patches and help improve this
+library. All patches are appreciated!

README.md

@@ -14,7 +14,10 @@
 - I still need to add implementation for chrome extension, works by injecting js for now
 - Main methods allow you to observe certain events with **`gmail.observe.on('lots_of_actions_here', callback())`** or similar **`gmail.observe.before(...)`** and **`gmail.observe.after(...)`**
 - Click on a method link to view more detailed docs
-- Create an issue/pull request for feedback, requests and fixes
+- Create an issue/pull request for feedback, requests and
+  fixes. See
+  [CONTRIBUTING.md](https://github.com/KartikTalwar/gmail.js/blob/master/CONTRIBUTING.md) for
+  more details.
 - Basic TypeScript-support from type-declarations file [gmail.d.ts](https://github.com/KartikTalwar/gmail.js/blob/master/src/gmail.d.ts).
 
 ### Installation