Bug 1749604 - new README-openbsd.md for creating openbsd vm. r=ng

Depends on D136030

Differential Revision: https://phabricator.services.mozilla.com/D136031

dom/media/webrtc/third_party_build/gn-configs/README-openbsd.md

@@ -0,0 +1,73 @@
+# Setup a Virtual Machine running OpenBSD to generate OpenBSD build files for libwebrtc.
+
+1. Download the OpenBSD 7.0 ISO [here](https://cdn.openbsd.org/pub/OpenBSD/7.0/amd64/install70.iso)
+
+2. Start by setting up a new VM. These steps reference VMWare fusion options,
+   but they should translate to other VM hosts.  During the VM creation steps,
+   using the following settings:
+   - VMWare - Other Linux 5.x kernel 64-bit
+   - VMWare - Legacy BIOS
+   - VMWare - Customize settings
+   - VMWare - Processors & Memory - 2 cores, 2048MB Memory
+   - VMWare - Hard Disk - 50.00 GB
+
+3. Start VM to begin the OpenBSD install process.
+
+   In general, choose the default options for everything unless listed below:
+   - select ```(I)nstall```
+   - allow root ssh - ```yes```
+   - disk - for more info see [here](https://azcrumpty.weebly.com/journal/easy-openbsd-partition-scheme)
+       - ```c``` # for custom
+       - ```z``` # for delete all
+       - ```a a``` # for add 'a' partition
+       - enter forward slash ('/') for the mount point
+       - ```q``` # to quit editor
+   - directory does not contain SHA256.sig. Continue without verification? ```yes```
+
+4. Upgrade to OpenBSD -current
+
+   Upgrading to -current is required for the minimum rust version.  For more
+   info, see [this](https://unix.stackexchange.com/questions/406870/how-to-follow-openbsd-current).
+
+   - login as root
+
+           ftp -o /bsd.rd https://cloudflare.cdn.openbsd.org/pub/OpenBSD/snapshots/amd64/bsd.rd
+           reboot
+
+   - at the bootloader prompt _quickly_ type
+
+           boot bsd.rd
+
+   - select ```U(Upgrade)``` at first prompt question, default to all other questions
+   - after upgrade completes allow reboot
+
+5. Upgrade and install necessary packages, add non-root user
+
+   - login as root
+
+         pkg_add -u
+         pkg_add sudo-- git-- bash-- mercurial-- m4-- py3-pip--
+         ln -sf /usr/local/bin/pip3.9 /usr/local/bin/pip3
+         ln -sf /usr/local/bin/python3 /usr/local/bin/python
+         visudo # uncomment line with '%wheel        ALL=(ALL) NOPASSWD: SETENV: ALL'
+
+         export NEW_USER={pick-a-user-name}
+         useradd -b /home -m -G wheel $NEW_USER
+         usermod -s /usr/local/bin/bash $NEW_USER
+         passwd $NEW_USER
+
+   - logout
+
+6. Clone mozilla-central repository and bootstrap
+
+   - login as {NEW_USER}
+
+           hg clone https://hg.mozilla.org/mozilla-central
+           (cd mozilla-central && ./mach bootstrap)
+
+   - for bootstrap:
+       - ignoring errors around glean-sdk
+       - select option ```1``` for all pkg_add prompts
+
+
+7. Continue with step 4 in README.md