Add link protocols.io for crystallization of SARS-CoV-2 Mpro

[maryhorsann]

Add link of protocols.io for crystallization of SARS-CoV-Mpro in the last TEP.
https://www.protocols.io/view/crystallization-of-sars-cov-2-mpro-kqdg326y7v25/v1

[jchodera]

The protocol is already posted, but the link to the plasmid at AddGene is missing, making this TEP useless:

Do we have an ETA for this plasmid being posted and a URL being available?