integrate btdigg search api

details on frostwire-desktop repo ([PR #376](https://github.com/frostwire/frostwire-desktop/pull/376))
I believe it's best if we leave these documentation outside of this patch and focus only on the patch at hand, then do another commit which adds these documents, or to add these documents on the github wiki.

We don't really mix readme's in the source code, they might as well be on some other folder related entirely to documentation as we don't want to by mistake package all these text files on the final binaries.