Add instructions for contributing

Author: sjsyrek

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing
+
+## Pull requests are welcome.
+
+All contributed code should conform to the following requirements:
+- functional in style, preferably following the example of haskell
+- written in a style similar to that of the rest of the library
+- presented in accordance with the standard style
+- checked with eslint
+- fully tested using mocha and should
+- fully covered, as shown in nyc/istanbul reports
+- fully documented using jsdoc style, with all additions, deletions, and changes reported in CHANGELOG.md
+- open source (including dependencies) on the same terms as the rest of this library