Add release guide and changelog

[jbolinger]

We need to publish an update for GAX and I noticed that there is no documentation (#61).

Can you do a quick sanity check or suggest improvements? I'll update as needed when I do an actual release os this is just a starting point (taken from the google-cloud-ruby).

[codecov-io]

Codecov Report

Merging #120 into master will decrease coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
- Coverage   99.54%   99.49%   -0.06%     
==========================================
  Files          19       19              
  Lines        1990     1990              
==========================================
- Hits         1981     1980       -1     
- Misses          9       10       +1

Impacted Files	Coverage Δ
spec/google/gax/bundling_spec.rb	99.69% <0%> (-0.31%)	:arrow_down:

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[blowmage]

Looks great to me. Thanks for this!