add removable-media info in snap description

[LyzardKing]

This should make it visible in the info shown in the software center.
It might be a good idea to also describe the process for connecting browsers,
or redirect to the relevant readme in github

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

Maybe we should add some text to http://docs.jabref.org/.

[LyzardKing]

Yes, I was looking at adding a section (or a link) in https://docs.jabref.org/getting-started
I just haven't had much time to actually do it...

Snap package:
installing web extensions...
adding external media...