Update git submodules
Update git submodules

* Update plugins/hooks from branch 'stable-2.15'
  to 9937b96271441f296a6557f98ed787960367b61f
  - Merge branch 'stable-2.14' into stable-2.15
    
    * stable-2.14:
      Add README
      Reword introduction in configuration documentation
      Fix formatting in configuration documentation
      Improve the "about" page
    
    Change-Id: I416f37eb11ede961f2b7afbcf0a47f79ce9fc906
    
  - Add README
    
    Add a README.md file in the repository root with the same content as
    the "About" page.
    
    Gitiles will render the README by default when the project is viewed
    on the googlesource site [1] and this will make the configuration
    documentation and list of supported hooks more easily accessible for
    users who have not installed the plugin yet in their Gerrit site.
    
    [1] https://gerrit.googlesource.com/plugins/hooks
    
    Change-Id: Ib2ecf818cbabaca8bfddf58e12cce99842a4f038
    
  - Reword introduction in configuration documentation
    
    Change-Id: I428167c7929a077386e281f4898e8f4693c315f2
    
  - Fix formatting in configuration documentation
    
    Change-Id: I7414451eea210df7cc3e4d865de5cc1286c7a4ef
    
  - Improve the "about" page
    
    Add a link to the git hooks documentation which gives more context
    about what is "not supported". Add links to the configuration and
    list of supported hooks.
    
    Change-Id: Ica629c70334951124bce439a4672fa28b40a2d61
    
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