Add Mozilla Code of Conduct

[Mozilla-GitHub-Standards]

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)

[Snuffleupagus]

Sorry, but I'm not convinced that merging this (or any) PR directly in this repository was a good idea; especially considering the README.

Can this be backed-out and fixed properly instead, i.e. by adding the new file to https://github.com/mozilla/pdf.js/tree/master/external/dist such that it's automatically included when the pdfjs-dist repo is updated? (There's also the risk that this change could be undone by a future update otherwise.)

[timvandermeij]

Yes, will do. All repositories got this PR, so after doing a few of them I apparently missed that this repository is maintained automatically.