Add or link build instructions to README

[krichter722]

Please provide build instructions in README.md or a link to them there since the build is more complex than mvn install (see #56).

This can somehow be combined with #24 and #10. They are however not about the specifics of the build process.

[turesheim]

I'm thinking maybe an addendum to the contribution guide would be better. https://github.com/eclipse/texlipse/blob/master/CONTRIBUTING.md