Update README.md

added instructions for a very useful feature of tmux. How to use two tmux default layouts.
Merge pull request #135 from morenoh149/patch-1

add instructions for using tmux built-in layouts
enable spellcheck on markdown files

added an autocommand that performs spellcheck on markdown files
Merge pull request #137 from morenoh149/patch-1

enable spellcheck on markdown files
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