Documents known limitations

[thinkingserious]

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I updated my branch with the master branch.
• I have added necessary documentation about the functionality in the appropriate .md file

Contributing to Twilio

All third party contributors acknowledge that any contributions they provide will be made under the same open source license that the open source project is provided under.

Short description of what this PR does:

• Documents known limitations of the Twilio CLI

[codecov]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files          26       26           
  Lines         789      789           
=======================================
  Hits          777      777           
  Misses         12       12

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