Add guide for testing patches.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

docs/DEV.md

@@ -1,7 +1,11 @@
-## Instructions for hacking on faasd itself
+## Instructions for building and testing faasd locally
 
 > Note: if you're just wanting to try out faasd, then it's likely that you're on the wrong page. This is a detailed set of instructions for those wanting to contribute or customise faasd. Feel free to go back to the homepage and pick a tutorial instead.
 
+Do you want to help the community test a pull request?
+
+See these instructions instead: [Testing patches](/docs/PATCHES.md)
+
 ### Pre-reqs
 
 > It's recommended that you do not install Docker on the same host as faasd, since 1) they may both use different versions of containerd and 2) docker's networking rules can disrupt faasd's networking. When using faasd - make your faasd server a faasd server, and build container image on your laptop or in a CI pipeline.

docs/PATCHES.md

@@ -0,0 +1,88 @@
+## Instructions for testing a patch for faasd
+
+### Launch a virtual machine
+
+You can use any kind of Linux virtual machine, Ubuntu 20.04 is recommended.
+
+Launch a cloud VM or use [Multipass](https://multipass.run), which is free to use an can be run locally. A Raspberry Pi 3 or 4 could also be used, but will need you to run `make dist` to cross compile a valid binary.
+
+### Copy over your SSH key
+
+Your SSH key will be used, so that you can copy a new faasd binary over to the host.
+
+```bash
+multipass launch \
+  --mem 4G \
+  -c 2 \
+  -n faasd
+
+# Then access its shell
+multipass shell faasd
+
+# Edit .ssh/authorized_keys
+
+# Add .ssh/id_rsa.pub from your host and save the file
+```
+
+### Install faasd on the VM
+
+You start off with the upstream version of faasd on the host, then add the new version over the top later on.
+
+```bash
+cd /tmp/
+git clone https://github.com/openfaas/faasd --depth=1
+cd faasd/hack
+./install.sh
+
+# Run the login command given to you at the end of the script
+```
+
+Get the multipass IP address:
+
+```bash
+export IP=$(multipass info faasd --format json| jq -r '.info.faasd.ipv4[0]')
+```
+
+### Build a new faasd binary with the patch
+
+Check out faasd on your local computer
+
+```bash
+git clone https://github.com/openfaas/faasd
+cd faasd
+
+gh checkout #PR_NUMBER_HERE
+
+GOOS=linux go build
+
+# You can also run "make dist" which is slower, but includes
+# a version and binaries for other platforms such as the Raspberry Pi
+```
+
+### Copy it over to the VM
+
+Now build a new faasd binary and copy it to the VM:
+
+```bash
+scp faasd ubuntu@$IP:~/
+```
+
+Now deploy the new version on the VM:
+
+```bash
+killall -9 faasd-linux; killall -9 faasd-linux ; mv ./faasd-linux /usr/local/bin/faasd
+```
+
+### Check it worked and test that patch
+
+Now run a command with `faas-cli` such as:
+
+* `faas-cli list`
+* `faas-cli version`
+
+See the testing instructions on the PR and run through those steps.
+
+Post your results on GitHub to assist the creator of the pull request.
+
+You can see how to get the logs for various components using the [eBook Serverless For Everyone Else](https://gumroad.com/l/serverless-for-everyone-else), or by consulting the [DEV.md](/docs/DEV.md) guide.
+