Update README.md.

Add the following information :
- distro tools and development packages needed ;
- distro 32-bit compatibility package needed (for 64-bit) ;
- how to run "gyp_chromium" to update build files after applying patches ;
- basic build step troubleshooting.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
Merge pull request #255 from Tarnyko/master

Update README.md.