Add LICENSE and update README.md

1. Add LICENSE for the "GNU GENERAL PUBLIC LICENSE Version 2"
2. Update README.md, add AUR installation, add description and link to
LICENSE
Revert "Add 2 new keywords of 'options' for syntax/PKGBUILD.vim"

This reverts commit 3841afe99d32d5c2f2dd78f8b99c62663c441e26.

For this commit results in 'conflicting files' error when installing package vim-pkgbuild.
Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/9
Add after/syntax/PKGBUILD.vim, update comment style

1. Add after/syntax/PKGBUILD.vim as supplement
   1.1 Add 2 new keywords for 'options' for syntax/PKGBUILD.vim
   1.2 Set current syntax variable
   Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/9
2. Update and unify comment style