Add alternate backend config and r53 details

[Aravindh-Raju]

Fixes #1263.

Changes in this pull request:

• Added details on how to use the alternate backend config instead of the global default one.
• Added details on how to use r53 as backend.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.02% 🎉

Comparison is base (dcfcf67) 92.21% compared to head (4535522) 92.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
+ Coverage   92.21%   92.24%   +0.02%     
==========================================
  Files         176      176              
  Lines        7682     7682              
  Branches      227      227              
==========================================
+ Hits         7084     7086       +2     
+ Misses        598      596       -2     

Files Changed	Coverage Δ
...cala/vinyldns/route53/backend/Route53Backend.scala	83.87% <ø> (ø)

... and 1 file with indirect coverage changes

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