Add unmaintained advisory for lmdb

[Jake-Shadle]

The lmdb crate https://github.com/danburkert/lmdb-rs has been unmaintained since roughly August 2018. This prompted mozilla to fork it and create their own crate lmdb-rkv...which is ironically, also unmaintained.

[tarcieri]

I agree the lmdb crate qualifies as unmaintained.

I think the wording of the advisory isn't great, particularly if the suggested replacement is also unmaintained. It does at least appear to be more recently maintained.

[Jake-Shadle]

Yes, it's the only replacement that I am aware of and does fix several bugs etc compared to the unmaintained one, however I was also going to open an advisory for lmdb-rkv as well after this is merged since yes, it hasn't had any activity in about 2 years, including an issue opened by mozilla themselves (well, a bot) due to travis becoming non-functional, which is why all of the PRs that have been languishing for those 2 years are all red. ☹️

[Jake-Shadle]

Yup, that was much nicer. 🙂