Update building documentation for macOS (#4850)

BUILDING.md

-See the [release](https://github.com/k3s-io/k3s/releases/latest) page for pre-built releases.
+**Note:** In case you are looking for the pre-built releases see the [release page](https://github.com/k3s-io/k3s/releases/latest).
+
+## Build k3s from source
+
+Before getting started, bear in mind that this repository includes all of Kubernetes history, so consider shallow cloning with (`--depth 1`) to speed up the process.
 
-The clone will be much faster on this repo if you do
 ```bash
 git clone --depth 1 https://github.com/rancher/k3s.git
 ```
 
-This repo includes all of Kubernetes history so `--depth 1` will avoid most of that.
+The k3s build process requires some autogenerated code and remote artifacts that are not checked in to version control. To prepare these resources for your build environment, run:
 
-The k3s build process requires some autogenerated code and remote artifacts that are not checked in to version control.
-To prepare these resources for your build environment, run:.
 ```bash
 mkdir -p build/data && make download && make generate
 ```
 
 To build the full release binary, you may now run `make`, which will create `./dist/artifacts/k3s`.
 
-To build the binaries using without running linting (ie; if you have uncommitted changes):
+To build the binaries using `make` without running linting (i.e.: if you have uncommitted changes):
+
 ```bash
 SKIP_VALIDATE=true make
 ```
 
-If you make any changes to go.mod and want to update the vendored modules, you should run the following before running `make`:
+In case you make any changes to [go.mod](go.mod), you should run `go mod tidy` before running `make`.
+
+### macOS considerations
+
+The shell scripts in charge of the build process (the ones behind `make`) rely on GNU utils (i.e., `sed`), [which slightly differ on macOS](https://unix.stackexchange.com/a/79357). So, if you need to build k3s on a macOS environment, it is suggested to use the virtual machine defined on this repository's [Vagrantfile](Vagrantfile) to perform the tasks mentioned above.
+
+To start the virtual machine, you will need [vagrant](https://www.vagrantup.com/) and [virtual box](https://www.virtualbox.org/) installed. Then prompt:
+
 ```bash
-go mod vendor && go mod tidy
+$ vagrant up
+[... vm provisioning logs ...]
 ```
 
-Kubernetes Source
------------------
+Once the virtual machine is provisioned, you should be able to ssh into it by doing `vagrant ssh` and perform any building task there:
 
-The source code for Kubernetes is in `vendor/` and the location from which that is copied
-is in `./go.mod`.  Go to the referenced repo/tag and you'll find all the patches applied
-to upstream Kubernetes.
+```bash
+$ vagrant ssh
+[... ssh connection logs ..]
+$ uname -a
+Linux k3s-0-alpine312 5.11.0-41-generic
+$ make
+[... k3s build logs ...]
+```
 
+All the artifacts built within the VM will be synchronized with the directory where the `vagrant up` command was issued. For vagrant related commands please refer to [its cli documentation](https://www.vagrantup.com/docs/cli).