docs: update documentation to refers to main branch instead of master

Author: xel1045

CONTRIBUTING.md

@@ -15,7 +15,7 @@ We accept contributions via Pull Requests on [Github](https://github.com/eXolnet
 
 - **Consider our release cycle** - We try to follow [SemVer v2.0.0](http://semver.org/). Randomly breaking public APIs is not an option.
 
-- **Create feature branches** - Don't ask us to pull from your master branch.
+- **Create feature branches** - Don't ask us to pull from your main branch.
 
 - **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.