README section for interop with google-http-client

[chingor13]

Adds an example using HttpCredentialsAdapter

Fixes googleapis/google-cloud-java#3013

[codecov]

Codecov Report

Merging #275 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #275      +/-   ##
============================================
- Coverage     78.23%   78.21%   -0.02%     
  Complexity      330      330              
============================================
  Files            21       21              
  Lines          1447     1446       -1     
  Branches        158      157       -1     
============================================
- Hits           1132     1131       -1     
  Misses          237      237              
  Partials         78       78

Impacted Files	Coverage Δ	Complexity Δ
...tp/java/com/google/auth/oauth2/UserAuthorizer.java	76.43% <0%> (-0.14%)	20% <0%> (ø)

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