docs: add list with alternative implementations

[mnm678]

Solves #128

It might be nice to clarify what implementations there are in other languages, but I couldn't find an active list of all the TUF implementations and don't think that this is the place to maintain that.

[coveralls]

Pull Request Test Coverage Report for Build 1531746689

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 233 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.5%) to 68.352%

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Files with Coverage Reduction	New Missed Lines	%
errors.go	9	0%
local_store.go	56	75.66%
repo.go	168	73.89%

Totals
Change from base Build 1356393585:	0.5%
Covered Lines:	2041
Relevant Lines:	2986

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💛 - Coveralls

[trishankatdatadog]

@joshuagl can you please 👀 and ✅ ?

[joshuagl]

Will take a look next week

[rdimitrov]

It might be nice to clarify what implementations there are in other languages, but I couldn't find an active list of all the TUF implementations and don't think that this is the place to maintain that.

I also think there's a need for such a list 👍

There's already an issue for the TUF website to have a list of some of the most meaningful implementations - theupdateframework/theupdateframework.io#27, so instead of having a list here maybe, it would be better to update the website and just refer to it? For example, a list of projects followed by a small description and maybe comparison info against some of the other similar projects)

[trishankatdatadog]

Can we either merge/update/close this? Thanks!

[rdimitrov]

@trishankatdatadog - hey, it seems that the DCO bot is complaining about one of your commits, so would you sign it and force push?

Another option is for @mnm678 or someone else (I can do it too if it's okay) to do an interactive rebase and squash all commits into a single signed one which hopefully will satisfy our bot.