Add documentation on how to run admin portal

[volokluev]

No description provided.

[codecov-commenter]

Codecov Report

Merging #2306 (5d3ba51) into master (07c8256) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2306   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files         557      557           
  Lines       25611    25611           
=======================================
  Hits        23763    23763           
  Misses       1848     1848           

---

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 07c8256...5d3ba51. Read the comment docs.

[lynnagara]

Thank you!

[evanh]

Can you add the build instructions to this as well?

volta install node
volta install yarn
make build-admin