1. Docs: Add CURRENT_BRANCH variable and code.
    
    Use a variable CURRENT_BRANCH to designate the AOSP branch to use for
    the changelog.  Update code to make the substitution.
    
    Bug: 34106192
    Test: python make_cdd.py --version 7.1 --branch nougat-mr1-dev
    
    Change-Id: Ia68946b91e61129929dd0dfb91d495d3c53b095b
    
  2. Docs: Update link for changelog.
    
    Change-Id: I08fbe5c66ac5d68c0dcddd8947e1ab8be78f8430
    
  3. Docs: Update link for changelog.
    
    Change-Id: I04057360d5d202c5e3f1fb1d058fc5dbe2fa9c3c
    
  4. Docs: Add CDD docs and the build script, and test examples
    
    Bug: 25199595
    This is based on the amended final CDD for M, hosted as commit
    1846a9622485855d572705a7972116caf0be3669 on the AOSP master branch.
    
    Change-Id: Ic3bd96cd652f7d7b13def03a4ca1f04645c34255