docs(readme): add watch option

[BBlackwo]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Tests added?	no
Docs updated?	yes
Fixed tickets	N/A
License	MIT

Added watch option to readme to more explicity show that the watch options sets several defaults.

When I upgraded from 1.* to 3.* I couldn't figure out why the grunt task never finished and moved to my next task. That's because the keepalive started being defaulted to true and I didn't realise it even after reading the docs.

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #157 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #157   +/-   ##
=======================================
  Coverage   66.35%   66.35%           
=======================================
  Files           8        8           
  Lines         211      211           
  Branches       53       53           
=======================================
  Hits          140      140           
  Misses         58       58           
  Partials       13       13

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[danez]

Thank you very much.