Update README.md

[blutack]

Just added a note to the readme about installation on OSX.
Libssh2 won't build by default because the OpenSSL headers were removed in OSX versions 10.10+. The fix is to install via Homebrew then set the environment so CMake can find them.

Again, thanks for xargo - it's awesome!

cargo build fails with the following:

--- stderr
fatal: Not a git repository (or any of the parent directories): .git
CMake Error at /usr/local/Cellar/cmake/3.1.1_1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:138 (message):
  Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the
  system variable OPENSSL_ROOT_DIR (missing: OPENSSL_INCLUDE_DIR)

[japaric]

Again, thanks for xargo - it's awesome!

❤️

Thanks for updating the docs 👍