Add an example for how to use the colored prompt.

[amjith]

Description

Add an example of how to use the colored prompt.

[codecov-io]

Codecov Report

Merging #1163 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1163      +/-   ##
==========================================
- Coverage   87.04%   86.89%   -0.15%     
==========================================
  Files          19       19              
  Lines        1806     1786      -20     
==========================================
- Hits         1572     1552      -20     
  Misses        234      234              

Impacted Files	Coverage Δ
pgcli/key_bindings.py	43.39% <0.00%> (-9.73%)	⬇️
pgcli/pgbuffer.py	65.00% <0.00%> (-1.67%)	⬇️
pgcli/completion_refresher.py	87.01% <0.00%> (-1.09%)	⬇️
pgcli/pgexecute.py	82.42% <0.00%> (-0.06%)	⬇️

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