TROUBLESHOOTING: introduce guide for common errors

Introduce a simple FAQ for common errors for people getting started with
the project.

TROUBLESHOOTING.md

+Troubleshooting libgit2 Problems
+================================
+
+CMake Failures
+--------------
+
+* **`Asked for OpenSSL TLS backend, but it wasn't found`**
+  CMake cannot find your SSL/TLS libraries.  By default, libgit2 always
+  builds with HTTPS support, and you are encouraged to install the
+  OpenSSL libraries for your system (eg, `apt-get install libssl-dev`).
+
+  For development, if you simply want to disable HTTPS support entirely,
+  pass the `-DUSE_HTTPS=OFF` argument to `cmake` when configuring it.