Add ~/.local/share/bash-completion/completions to the README

[postmodern]

~/.local/share/bash-completion/completions is the XDG directory for bash completions installed by the user.

[DannyBen]

Thanks.

1. Do you have a reference that lists this directory as a common location?
2. We should probably also add it to the list of directories that are used by the installer:
   

   completely/lib/completely/installer.rb

   Lines 11 to 15 in 54dd259

   	@target_directories ||= %W[
   	/usr/share/bash-completion/completions
   	/usr/local/etc/bash_completion.d
   	#{Dir.home}/.bash_completion.d
   	]

[postmodern]

@DannyBen there are a few Google search results:

1. https://github.com/scop/bash-completion/blob/master/README.md#faq
2. A StackOverflow comment explaining when .local/share/bach-completion/completion was added to replace ~/.bash_completion.d.
   https://serverfault.com/a/831184

[DannyBen]

Thanks.

Merged and added to the installer as well.