Add some more dependencies to doc/HOWTO.txt.
Add some more dependencies to doc/HOWTO.txt.

We recently added a dependency on libgmp-dev...

Yawning/obfsproxy,catinred2/obfsproxy,infinity0/obfsproxy,NullHypothesis/obfsproxy,Yawning/obfsproxy-wfpadtools,sunsong/obfsproxy,masterkorp/obfsproxy,david415/obfsproxy,qdzheng/obfsproxy,isislovecruft/obfsproxy