docu(share_plus): document Facebook sharing inability (#413)

[Boehrsi]

Description

Updated the documentation to state the inability to share data to Facebook apps. As this PR contains only documentation changes I haven't updated the change log.

Related Issues

Closes #413

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[miquelbeltran]

LGTM! As I understand this is an issue for Android and iOS right? could you specify that (although may be obvious for some)

[Boehrsi]

Had to force push, as I discovered (happily before merging this PR) that I initially edited the readme file of the package_plus project and not sharing_plus. I fixed this and also added clarification in regards to the platforms.