add contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contribution guidelines
+
+## For Issues:
+
+0. Check for **similar issues** first!
+0. Provide as **many details** as possible
+
+## For Pull Requests:
+
+0. Indent using **2 spaces**
+0. Use **double quotes** for strings
+  * Exception: `'"'`
+0. Write only **one expression per line**
+  * Bad: `foo(); bar();`
+0. Write **valid JavaScript**, a code linter is recommended!
+0. **Explain code** that is not obvious (comment above)
+0. If you add new functions, **add documentation** (comment above)
+  * Wrap function arguments in `+` signs (e.g. `the +type+ is used for…`)
+0. If you add new functions, **add tests** (test/test.js)
+0. Run **tests** (`npm test`)
+0. Make sure as much of your code as possible is **covered by tests**
+0. Double check that your pull request is **awesome** :)