README: link to build instructions, slight polish to Markdown

I found it difficult to find the instructions for building MacVim from source, but very easy to build it once I found the instructions.

Hopefully, this will help others find these helpful instructions.

Author: ilyagr

README.md

@@ -1,9 +1,11 @@
 Vim - the text editor - for macOS
 
 
-- MacVim homepage https://macvim-dev.github.io/macvim
+- MacVim homepage: <https://macvim-dev.github.io/macvim>
 
-- Download the latest version from [Releases](https://github.com/macvim-dev/macvim/releases/latest).
+- Download the latest version from [Releases](https://github.com/macvim-dev/macvim/releases/latest)
+
+- How to [build MacVim from source](src/MacVim/README) 
 
 - Vim README: [README_vim.md](README_vim.md)