add deprecation notice to README

[cgdibble]

No description provided.

[codecov-io]

Codecov Report

Merging #36 (5ab66a3) into master (fa6cf37) will decrease coverage by 0.92%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   94.93%   94.01%   -0.93%     
==========================================
  Files           9        9              
  Lines         375      384       +9     
==========================================
+ Hits          356      361       +5     
- Misses         19       23       +4     

Impacted Files	Coverage Δ
src/executor.js	87.62% <60.00%> (-3.29%)	⬇️
src/profile.js	91.66% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3ded32e...5ab66a3. Read the comment docs.