Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# How to contribute
+
+## GIT
+
+### Message Format
+There are a few basic rules that makes Git commit messages more readable on the terminal and GitHub. The most important rule is use the first line as a summary and to keep it short.
+
+According to [Pro Git: Chapter 5.2](http://progit.org/book/ch5-2.html#Commit-Guidelines) we use this template:
+
+    Short (50 chars or less) summary of changes
+
+    More detailed explanatory text, if necessary.  Wrap it to about 72
+    characters or so.  In some contexts, the first line is treated as the
+    subject of an email and the rest of the text as the body.  The blank
+    line separating the summary from the body is critical (unless you omit
+    the body entirely); tools like rebase can get confused if you run the
+    two together.
+
+    Further paragraphs come after blank lines.
+
+    - Bullet points are okay, too
+
+    - Typically a hyphen or asterisk is used for the bullet, preceded by a
+      single space, with blank lines in between, but conventions vary here
+
+### Branching
+We want to keep our commit histories clean where possible. 
+- master / dev / feature-<name> / hotfixex / release-<version> for branches.
+- merging with `--no-ff` option;
+- remember about `git rebase`;