add CONTRIBUTING.md

Author: Gelio

.npmignore

@@ -8,3 +8,4 @@ tsconfig.json
 tslint.json
 .prettierrc
 .editorconfig
+CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing guidelines
+
+Anyone is welcome to contribute to this project. Filing issues with detected bugs or false positives
+is appreciated, as is opening pull requests with fixes to those.
+
+Before opening a pull request, make sure that all the tests pass by running
+
+```sh
+npm run test
+```
+
+It would be great if you added new test cases that cover the functionality you added/fixed in the
+pull request.
+
+## Information for developers
+
+Creating TSLint rules may seem complex in the beginning, but it is actually pretty easy once you
+understand the underlying concepts.
+
+Each Typescript file is parsed into an Abstract Syntax Tree (AST). It is a tree structure that
+contains nodes like statements, expressions, identifier that appear in the file being linted. The
+rule can then analyze the AST and report rule violations.
+
+Check the [TSLint documentation for developing TSLint rules](https://palantir.github.io/tslint/develop/custom-rules/)
+for more information on the subject.
+
+[AST Explorer](https://astexplorer.net/) is an extremely useful tool when working with and exploring ASTs. It
+supports different parsers, so when developing TSLint rules make sure to pick the _typescript_
+parser from the settings bar at the top.