Reference libraries-bom in docs + Add .gitignore

[athakor]

towards 6668

[codecov]

Codecov Report

Merging #16 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #16      +/-   ##
===========================================
+ Coverage     76.93%   77.2%   +0.26%     
- Complexity      119     140      +21     
===========================================
  Files             6       6              
  Lines          1006    1079      +73     
  Branches          3      17      +14     
===========================================
+ Hits            774     833      +59     
- Misses          212     223      +11     
- Partials         20      23       +3

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/iot/v1/DeviceManagerClient.java	59.92% <0%> (-0.51%)	79% <0%> (+19%)
...e/cloud/iot/v1/stub/DeviceManagerStubSettings.java	84.42% <0%> (+0.21%)	29% <0%> (+1%)	⬆️
...oogle/cloud/iot/v1/stub/GrpcDeviceManagerStub.java	97.66% <0%> (+0.22%)	26% <0%> (+1%)	⬆️

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