rpc: Add testing-related documentation overview

[thanethomson]

Closes #597

Rendered README

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #604 into master will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #604   +/-   ##
======================================
  Coverage    41.3%   41.3%           
======================================
  Files         163     163           
  Lines       11317   11311    -6     
  Branches     2558    2559    +1     
======================================
  Hits         4682    4682           
+ Misses       6264    6258    -6     
  Partials      371     371           

Impacted Files	Coverage Δ
tendermint/src/time.rs	59.1% <0.0%> (+6.4%)	⬆️

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[ebuchman]

Super helpful, especially for surfacing more questions, thanks!