CONTRIBUTING: document build requirements

Signed-off-by: Hidde Beydals <[email protected]>

CONTRIBUTING.md

@@ -27,6 +27,40 @@ to join the conversation (this will also add an invitation to your
 Google calendar for our [Flux
 meeting](https://docs.google.com/document/d/1l_M0om0qUEN_NNiGgpqJ2tvsF2iioHkaARDeh6b70B0/edit#)).
 
+### Installing required dependencies
+
+The dependency [libgit2](https://libgit2.org/) needs to be installed to be able
+to run source-controller or its test-suite locally (not in a container).
+
+In case this dependency is not present on your system (at the expected
+version), the first invocation of a `make` target that requires the
+dependency will attempt to compile it locally to `hack/libgit2`. For this build
+to succeed; CMake, OpenSSL 1.1 and LibSSH2 must be present on the system.
+
+Triggering a manual build of the dependency is possible as well by running
+`make libgit2`. To enforce the build, for example if your system dependencies
+match but are not linked in a compatible way, append `LIBGIT2_FORCE=1` to the
+`make` command.
+
+#### macOS
+
+```console
+$ # Ensure libgit2 dependencies are available
+$ brew install cmake [email protected] libssh2 pkg-config
+$ LIBGIT2_FORCE=1 make libgit2
+```
+
+#### Linux
+
+```console
+$ # Ensure libgit2 dependencies are available
+$ pacman -S cmake openssl libssh2
+$ LIBGIT2_FORCE=1 make libgit2
+```
+
+**Note:** Example shown is for Arch Linux, but likewise procedure can be
+followed using any other package manager, e.g. `apt`.
+
 ### How to run the test suite
 
 Prerequisites: