Centralise Contributor docs, add Dev doc

	As part of fluxcd/.github#4, create DEVELOPMENT.md
	Will link to this doc from fluxcd.io/contributing
	so new folks can find this easily.

Signed-off-by: Daniel Holbach <[email protected]>

Author: Daniel Holbach

CONTRIBUTING.md

@@ -0,0 +1,47 @@
+# Development
+
+> **Note:** Please take a look at <https://fluxcd.io/docs/contributing/flux/>
+> to find out about how to contribute to Flux and how to interact with the
+> Flux Development team.
+
+## 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, Docker, 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
+
+You can run the unit tests by simply doing
+
+```bash
+make test
+```