Add a doc on how to migrate from SVN to gitlab.

doc/workflow/migrating_from_svn.md

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+# Migrating from SVN to GitLab
+
+SVN stands for Subversion and is a version control system (VCS).
+Git is a distributed revision control and source code management (SCM) system.
+
+There are some major differences between the two, for more information consult your favourite search engine.
+
+Git has tools for migrating SVN repositories to git, namely `git svn`. You can read more about this at
+[git documentation pages](http://git-scm.com/book/en/Git-and-Other-Systems-Git-and-Subversion).
+
+Apart from the [official git documentation](http://git-scm.com/book/en/Git-and-Other-Systems-Migrating-to-Git) there is also
+user created step by step guide for migrating from SVN to GitLab.
+
+[Benjamin New](https://github.com/leftclickben) wrote [a guide that shows how to do a migration](https://gist.github.com/leftclickben/322b7a3042cbe97ed2af). Mirrors can be found [here](https://gitlab.com/snippets/2168) and [here](https://gist.github.com/maxlazio/f1b593b0d00aa966e9ca).
+
+## Contribute to this guide
+We welcome all contributions that would expand this guide with instructions on how to migrate from other version control systems.