add requirement doc for FreeBSD (#35704)

* add freebsd requirements

* corrections

* format

* feedback from review

* more cleanup

Co-authored-by: Tomas Weinfurt <[email protected]>

Author: wfurt

docs/workflow/README.md

@@ -10,7 +10,7 @@ The repo can be built for the following platforms, using the provided setup and
 | x86   | ✔ |          |          |          |
 | ARM   | ✔ | ✔ |          |          |
 | ARM64 | ✔ | ✔ |          |          |
-|       | [Requirements](requirements/windows-requirements.md) | [Requirements](requirements/linux-requirements.md) | [Requirements](requirements/macos-requirements.md) |
+|       | [Requirements](requirements/windows-requirements.md) | [Requirements](requirements/linux-requirements.md) | [Requirements](requirements/macos-requirements.md) | [Requirements](requirements/freebsd-requirements.md)
 
 Before proceeding further, please click on the link above that matches your machine and ensure you have installed all the prerequisites for the build to work.
 

docs/workflow/requirements/freebsd-requirements.md

@@ -0,0 +1,69 @@
+Requirements to build dotnet/runtime on FreeBSD
+=====================
+
+This guide will walk you through the requirements needed to build dotnet/runtime on FreeBSD. We'll start by showing how to set up your environment from scratch.
+Since there is no official build and FreeBSD package, native build on FreeBSD is not trivial. There are generally three options, sorted by ease of use:
+- cross-compile on Linux using Docker
+- cross-compile on Linux using Toolchain
+- build on FreeBSD
+
+
+Environment
+===========
+
+These instructions were validated for and on FreeBSD 11.3 and 12.1.
+
+Build using Docker on Linux
+---------------------------
+
+This is similar to [Linux](linux-requirements.md) instructions. https://github.com/dotnet/dotnet-buildtools-prereqs-docker repro provides images
+with all needed prerequisites to build. As the example bellow may become stale, https://github.com/dotnet/versions/blob/master/build-info/docker/image-info.dotnet-dotnet-buildtools-prereqs-docker-master.json offers list of latest Docker tags.
+
+```sh
+TAG=mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-freebsd-11-20200430154008-a84b0d2
+docker run --rm --volume $(pwd):$(pwd) --workdir $(pwd) --env ROOTFS_DIR=/crossrootfs/x64 -ti  $TAG ./build.sh -cross -FreeBSD
+```
+
+Build using Toolchain Setup
+---------------------------
+To build FreeBSD images, prerequisites described in [Linux](linux-requirements.md) are needed. Additionally, crossrootfs  for FreeBSD needs to be constructed.
+In order to successfully build FreeBSD crossrootfs, few more packages needs to be installed. Following example is for Ubuntu 18:
+```sh
+apt-get install -y libbz2-dev libz-dev liblzma-dev libarchive-dev libbsd-dev
+```
+With prerequisites for crossrootfs one can run:
+```sh
+./eng/common/cross/build-rootfs.sh freebsd11 $(pwd)/rootfs/freebsd
+```
+After that, FreeBSD build can be started by running
+```
+ROOTFS_DIR=$(pwd)/rootfs/freebsd ./build.sh -cross -os FreeBSD
+```
+
+
+Building on FreeBSD
+-------------------
+
+Building dotnet/runtime depends on several tools to be installed.
+
+Install the following packages:
+
+- cmake
+- autoconf
+- automake
+- libtool
+- icu
+- libunwind
+- lttng-ust
+- krb5
+- openssl (optional)
+
+The lines to install all the packages above using package manager.
+
+```sh
+sudo pkg install --yes libunwind icu libinotify lttng-ust krb5 cmake autoconf automake openssl
+```
+
+Additionally, working dotnet cli with SDK is needed. On other platforms this would be downloaded automatically during build but it is not currently available for FreeBSD.
+It needs to be built once on supported platform or obtained via community resources.
+