Add instructions for changing branches with PKGBUILD. (#613)

Co-authored-by: Lord Valen <lord_valen@protonmail.com>

README.md

@@ -150,6 +150,18 @@ git clone https://aur.archlinux.org/neovide-git.git
 cd neovide-git
 makepkg -si
 ```
+
+To install a non-default branch:
+
+```sh
+git clone https://aur.archlinux.org/neovide-git.git
+cd neovide-git
+nvim PKGBUILD
+:%s/l}/l}#branch=branch-name-here/
+:wq
+makepkg -si
+```
+
 Note: Neovide requires that a font be set in `init.vim` otherwise errors might be encountered.
 See [#527](https://github.com/Kethku/neovide/issues/527)