Document limitations

[cketti]

This library doesn't support…

• attachments (not supported by mailto URI)
• names that go with email addresses, e.g. John Doe <j.doe@example.org> (not sure if support possible)
• non-text/plain mail bodies; see also Update documentation: HTML emails are not supported #1 (not supported by mailto URI)

Update the documentation to include this information and point to alternatives if available, e.g. ACTION_SEND.