Added initial CONTRIBUTING.md (#425)

CONTRIBUTING.md

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+# Issue descriptions
+
+Please see the [issue template](ISSUE_TEMPLATE.md) for how to write a good
+issue description. In short, it should contain the following:
+
+1. Describe the issue in detail, include steps to reproduce the issue
+2. Include a minimal working example
+3. Include a minimal vimrc file
+4. If you have a `.latexmkrc` file, please mention it and provide the relevant
+   content
+
+# Guide for code contributions
+
+## Branch model
+
+vimtex is developed mainly through the master branch, and pull requests should
+be [fork based](https://help.github.com/articles/using-pull-requests/).
+
+## Code style used with vimtex
+
+When submitting code for vimtex, please adhere to the following standards:
+
+- Use `shiftwidth=2` - no tabs!
+- Write readable code
+  - Break lines for readability
+    - Line should not be longer than 80 columns
+  - Use comments:
+    - For complex code that is difficult to understand
+    - Simple code do not need comments
+  - Use (single) empty lines to separate logical blocks of code
+  - Use good variable names
+    - The name should indicate what the variable is/does
+    - Variable names should be lower case
+    - Local function variables should be preceded with `l:`
+- Use markers for folding (see code for examples)
+