Add Mozilla Code of Conduct

[Mozilla-GitHub-Standards]

Fixes #275

As of January 1 2019, Mozilla requires that all GitHub projects include this CODE_OF_CONDUCT.md file in the project root. The file has two parts:

1. Required Text - All text under the headings Community Participation Guidelines and How to Report, are required, and should not be altered.
2. Optional Text - The Project Specific Etiquette heading provides a space to speak more specifically about ways people can work effectively and inclusively together. Some examples of those can be found on the Firefox Debugger project, and Common Voice. (The optional part is commented out in the raw template file, and will not be visible until you modify and uncomment that part.)

If you have any questions about this file, or Code of Conduct policies and procedures, please see Mozilla-GitHub-Standards or email Mozilla-GitHub-Standards+CoC@mozilla.com.

(Message COC002)

[codecov-io]

Codecov Report

Merging #276 into master will increase coverage by 0.09%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #276      +/-   ##
============================================
+ Coverage     35.99%   36.08%   +0.09%     
- Complexity      395      396       +1     
============================================
  Files           125      125              
  Lines          3273     3273              
  Branches        509      509              
============================================
+ Hits           1178     1181       +3     
+ Misses         1998     1995       -3     
  Partials         97       97

Impacted Files	Coverage Δ	Complexity Δ
.../org/mozilla/focus/search/SearchEngineManager.java	57.6% <0%> (+3.26%)	13% <0%> (+1%)	:arrow_up:

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