Merge pull request #292 from lucafavatella/guidelines

Mention guidelines

.editorconfig

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+root = true
+
+[*]
+indent_style = space
+indent_size = 2

CONTRIBUTING.md

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+# Erlang Guidelines
+
+This project follows [Inaka's Erlang
+guidelines](https://github.com/inaka/erlang_guidelines).
+
+Rules are not applied strictly, and the main rule is [maintaining the
+existing
+style](https://github.com/inaka/erlang_guidelines#maintain-existing-style):
+
+> When editing a module written by someone else, stick to the style in
+  which it was written. If a project has an overall style, stick to
+  that when writing new modules as well.
+
+Some of the guidelines are codified as [editor-agnostic
+configuration](http://editorconfig.org/) in the `.editorconfig` file.