Documentation: Add instructions for setting up self-hosted runners

Documentation/SelfHostedRunners.md

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+# SerenityOS self-hosted runner setup instructions
+
+## Requirements
+
+Since these self hosted-runners are supposed to be a more performant alternative to the GitHub-provided runners, the bare minimum requirements are GitHub's own Linux runner [hardware specification](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources) as well as guaranteed uptime.
+
+As for recommended requirements, listed below are the specifications of the current SerenityOS runners, roughly matching these would eventually make running performance-regression related tests on these easier. (But this is not a hard requirement, as GitHub offers the ability to selectively choose which self-hosted runners run which workflow)
+
+#### IdanHo runner:
+ - Ryzen 5 3600 - 12 cores w/ KVM support
+ - 64GB of RAM
+ - 512GB of SSD space
+###### This runner can be split into 2 runners with half the cores/RAM/space if needed. 
+
+## Setup
+
+These instructions assume the OS installed is Ubuntu 20.04 (Focal), so they might not be compatible with other Linux flavours.
+
+### Install base dependencies
+```shell
+add-apt-repository ppa:canonical-server/server-backports
+apt update
+apt install build-essential make cmake clang-format-11 gcc-10 g++-10 libstdc++-10-dev libgmp-dev ccache libmpfr-dev libmpc-dev ninja-build e2fsprogs qemu-utils qemu-system-i386 wabt
+```
+### Force usage of GCC 10
+```shell
+update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 100 --slave /usr/bin/g++ g++ /usr/bin/g++-10
+```
+### Create a new user account named 'runner'
+```shell
+adduser runner
+```
+### Give it password-less sudo capabilities by adding the following line to /etc/sudoers:
+```shell
+runner ALL=(ALL) NOPASSWD:ALL
+```
+### Add it to the kvm access group (if available):
+```shell
+adduser runner kvm
+```
+### Switch to the new user and then create a workspace folder in its home directory:
+```shell
+mkdir actions-runner && cd actions-runner
+```
+### Download the latest version of actions-runner-linux-x64 from https://github.com/rust-lang/gha-runner/releases
+```shell
+curl -o actions-runner-linux-x64-X.X.X.tar.gz -L https://github.com/rust-lang/gha-runner/releases/download/vX.X.X-rust1/actions-runner-linux-x64-X.X.X-rust1.tar.gz
+```
+### Extract the tar archive
+```shell
+tar xzf ./actions-runner-linux-x64-X.X.X.tar.gz
+```
+### Link the runner to the repository
+```shell
+./config.sh --url https://github.com/SerenityOS/serenity --token INSERT_SECRET_TOKEN_HERE
+```
+### Configure the runner to protect against malicious PRs by adding the following line to .env:
+```shell
+RUST_WHITELISTED_EVENT_NAME=push
+```
+### Configure the maximum runner threads by adding the following line to .env:
+```shell
+MAX_RUNNER_THREADS=XXX
+```
+If you are setting up multiple runners on the same machine, this setting can be used to divvy up the cores, if youre only setting up one runner, this can just be set to the server's core count
+### Install the runner as a service
+```shell
+sudo ./svc.sh install
+```
+### Start the runner
+```shell
+sudo ./svc.sh start
+```