docs: improve contributing documentation (siderolabs#418)

Signed-off-by: Tim Jones <[email protected]>

CONTRIBUTING.md

@@ -30,7 +30,49 @@ conform enforce --commit-msg-file $1
 EOF
 ```
 
+In addition, all commits should be signed by the committer using `git commit -s` which should produce a commit
+message with `Signed-off-by: Your Name <your@email>`. It is not necessary to cryptographically sign commits
+with GPG.
+
 ### Pull Requests
 
-To avoid multiples CI runs, please ensure that you are running a full build before submitting your PR.
+To avoid multiples CI runs, please ensure that you are running a full build before submitting your PR, and
+branches should be squashed to a single commit.
+
+## Developing
+
+To start developing for Talos you will need at least `GNU Make` and `Golang` v1.11.
+
+From there you can bootstrap the rest of the toolchain and install Golang dependencies with the following commands:
+
+```bash
+GO111MODULE=on go get
+make ci
+```
+
+# Make Targets
+
+In the `Makefile` there are a variety of targets, the most common are:
 
+* `docs` builds the Hugo-based documentation site.
+* `kernel` creates the `vmlinuz` Linux kernel executable.
+* `initamfs` creates the `initramfs.xz` initial RAMdisk filesystem.
+* `image-vanilla` creates the `image.raw` file that can be used as a image volume for VMs.
+* `osctl-linux-amd64` and `osctl-darwin-amd64` make the `osctl` CLI tool for Linux & OSX respectivly.
+* `rootfs` creates an archive of the root filesystem preloaded with all the components needed to launch Talos & Kubernetes.
+
+# Buildkit
+
+Talos uses Moby [buildkit](https://github.com/moby/buildkit) for concurrent and cache-efficient builds.
+By default, a buildkit service is started locally, but if you want to offload the builds to another server,
+you can start a buildkit service with the following command:
+
+```bash
+docker run --detach --privileged --restart always --publish 1234:1234 moby/buildkit --addr tcp://0.0.0.0:1234
+```
+
+Then using the `BUILDKIT_HOST` environment variable before running any `make` target, E.G.
+
+```bash
+BUILDKIT_HOST=tcp://192.168.1.50:1234 make initramfs
+```