Add new BUILDING.md with "Building from Source" content from k3s docs - We migrated "Building from Source" on k3s docs to this GH file. - Make some small tweaks
Add new BUILDING.md with "Building from Source" content from k3s docs
- We migrated "Building from Source" on k3s docs to this GH file.
- Make some small tweaks

rancher/k3s,rancher/k3s