I keep my to-do list on Trello. If you want to contribute to the project, anything in the 'To-Do' list with a 'bug' or 'quick-fix' label is probably a good place to start. Many of these features can be implemented fairly straightforwardly by writing a new plugin.
Another good way of getting up to speed with the project would be to contribute to the documentation.
If you want to do something from the 'Ideas' list, you feel one of the tasks requires more elaboration, or you want some help with the code-base, you're welcome to contact me via email.
Contexts is developed using Test-Driven-Development. Any new features should have accompanying tests (written with Contexts) - see the existing tests for examples.
Once your feature is finished and tested, simply submit a pull request to this repository.