It's one of the links to EDMs that
@MeSci found. I'm sure you're right that he wasn't aware of the background to the CMRC.
Here's another one he signed in 2010:
http://www.parliament.uk/edm/2010-12/778 (edited as I posted the wrong link first time).
Apparently, Jeremy Corbyn has been very supportive of issues face by constituents with ME for many years, and in the past was given a copy of the book Lost Voices from a hidden illness (which led to the film Voices from the Shadows) and the IiME Conference DVDs so his agreement to speak at the next Invest in ME Research Conference hasn't come out of the blue.