Update list of plugins in readme

We have added a few plugins that were not listed in the README.md file:

  - commit_msg/hard_tabs.rb
  - commit_msg/single_line_subject.rb
  - pre_commit/ruby_style.rb

This was easily accomplished with vim, by executing:

  :r ! tree lib/overcommit/plugins

Change-Id: I96582b0318ef785af9cd158c1a1330878f295f85
Reviewed-on: https://gerrit.causes.com/25177
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>