Add guide for testing patches.

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

docs/PATCHES.md

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+## 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.
+