Add links to replacement "plus" plugins

[TheLastGimbus]

I though adding Github links to those new "plus" plugins would be nice, since the original ones do have link :)

List which issues are fixed by this PR. You must list at least one issue.

• I think none

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[TheLastGimbus]

CI failing is irrelevant 👌

By the way, why are you guys not participating in hacktoberfest 🤔 ? All it takes is add "hacktoberfest" tag to repo 👍 🚀

[stuartmorgan]

Thanks for the contribution, but linking only to the published package was an intentional choice; how the source for those plugins is organized is an implementation detail that can change (and in fact already has at least once since this table was created), while the pub.dev links are stable. The pub.dev pages in turn link to the source, via links that are controlled by the plugin developers and will thus presumably be updated for changes in structure, so there is already a straightforward way for people to get to the source that doesn't involve fragile links from this repo.

By the way, why are you guys not participating in hacktoberfest

#4408 (comment)

[TheLastGimbus]

Oki-doki, understandable 💯

our experience with the event has not been positive

oof 😞

It was mostly completely pointless README changes

😅 😅 😅 Don't know what you're talking about 😅 😅 😅

[stuartmorgan]

It was mostly completely pointless README changes

😅 😅 😅 Don't know what you're talking about 😅 😅 😅

To be clear, this is not the kind of thing I was referring to. This change had a goal, it's just not something we want to do for logistical reasons. I'm taking about changes that were change just for the sake of creating a PR, with no possible benefit to anyone.