Update documentation to match the consistent Remixer Docs structure

[miguelandres]

• Replaced logo with a smaller version embedded in an H1.
• Added screenshot to the right.
• Renamed Getting started to Using Remixer
• Added sections in README.md for Using Remixer and Contributing instead of them being just parts of a table of contents.
• Adds documentation on how to configure the Firebase Remote Controller from the point of view of the Android Remixer Framework
• Adds an explanation on when you may not want/need a RemixerFragment.

[codecov-io]

Codecov Report

Merging #138 into develop will decrease coverage by 0.03%. The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop     #138      +/-   ##
=============================================
- Coverage      56.29%   56.25%   -0.04%     
  Complexity       233      233              
=============================================
  Files             54       54              
  Lines           1526     1527       +1     
  Branches         150      150              
=============================================
  Hits             859      859              
- Misses           611      612       +1     
  Partials          56       56

Impacted Files	Coverage Δ	Complexity Δ
...emixer/storage/FirebaseRemoteControllerSyncer.java	0% <0%> (ø)	0 <0> (ø)	:x:

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