You mention that you need to know the framework in order to be able to contribute...
Maybe you just need to get better at understand code you have never seen before.
If it is the case you should try to read more code, understand what it does and why it does it in that specific way.
Since read code for the sake of read code is boring, write code is way more fun, you need a reason.
Right now I am working to a little side project where I am trying to build a community of open source developer and companies so that companies will be able to hire via open source : http://siscia.github.io/open-hire/
In order to build the community of developers I am trying to write high level documentation for popular open source project, more than documentation is a code walk through, explain what is going on and why.
Today it was my first day, but I really learned a TON from a well written python project and I barely scratched the surface.
I would suggest you to do the same, pick any open source project and write high level documentation for such project.
What project to choose is up to you, ideally you should know the language, or at least understand the syntax.
A cool one to do could be, liquid template.
You can find other nice (IMHO) open source project here: https://github.com/siscia/open-hire/blob/gh-pages/_data/lang...
Also, if you are proud of the documentation you wrote I would be extremely happy to publish your work :)
(Feel free to write me, my email is all over the linked page ;) )