Record the number of families in each release in a standard way

[AntonPetrov]

• Update section 5 of the Rfam FTP README file with historic info for releases 14.1-14.7. To get the old info one can parse the families table in mysql dumps for each release. Note: I've also added Update section 5 of FTP Readme to the 14.8 release checklist.
• Consider creating an easily parseable file (tsv, csv) with release stats
• Bonus points: include number of sequences, alignment size, mean percent identity (PID), Min PID, number of basepairs, C+G content, covariation, presence of pseudoknots. Many of these metrics can be obtained by running esl-alistat on the seed files.

For more background and to cite a tweet when done: https://twitter.com/YannPonty/status/1488136823771705344

[emmaco]

File now available with version, date and number of families https://ftp.ebi.ac.uk/pub/databases/Rfam/CURRENT/release_stats.tsv
To be updated each release.
Hopefully can expand to include other useful info, as suggested.