Document "Limitations" and "Workarounds".

[taiki-e]

pin-project-lite has some limitations and workarounds that are only described in the issue or code. It would be nice if we could provide them as part of the documentation.

Issues:

• no support for multiple trait bounds #2 (workaround: https://github.com/taiki-e/pin-project-lite/issues/2#issuecomment-745190950)
• no interoperability with other field attributes #3 (workaround: https://github.com/taiki-e/pin-project-lite/issues/3#issuecomment-745194112)
• https://github.com/taiki-e/pin-project-lite/issues/3#issuecomment-758323290
• https://github.com/taiki-e/pin-project-lite/issues/52

Code comments:

https://github.com/taiki-e/pin-project-lite/blob/416be96f7777862c68b567c92a91887f69a8c2b3/src/lib.rs#L401-L408

https://github.com/taiki-e/pin-project-lite/blob/6f7a40dbcc971d5e94d8fb81592c6fab66d898a4/tests/test.rs#L577-L584