License command documentation and cache

[nicholasks]

• Adding documentation regarding new license command, also added to history.
• Setting cache for the licenses db to one week (7 days).

[codecov]

Codecov Report

Merging #326 (df6c5c3) into master (2e5b46b) will increase coverage by 0.32%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   68.56%   68.88%   +0.32%     
==========================================
  Files           8        8              
  Lines         474      495      +21     
==========================================
+ Hits          325      341      +16     
- Misses        149      154       +5     

Impacted Files	Coverage Δ
safety/cli.py	50.00% <12.50%> (-6.85%)	⬇️
safety/constants.py	100.00% <100.00%> (ø)
safety/errors.py	100.00% <100.00%> (ø)
safety/safety.py	92.96% <100.00%> (+6.78%)	⬆️

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[rafaelpivato]

LGTM