Add --help output to README

[clbarnes]

Additionally, refer to environment variables which can be used to configure login options.

[codecov-io]

Codecov Report

Merging #1164 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1164   +/-   ##
=======================================
  Coverage   87.04%   87.04%           
=======================================
  Files          19       19           
  Lines        1806     1806           
=======================================
  Hits         1572     1572           
  Misses        234      234           

---

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 2a28399...3f15c73. Read the comment docs.

[laixintao]

Thanks, I think it's a good idea about showing --help in readme.md in case of others miss alias_dsn feature :) cc @amjith @j-bennet