docs: Update README to include feature proposal instructions

[khawarizmus]

Added information on contributing to proposals.

[cahofmeyr]

We should maybe mention something about submitting or upvoting ideas on https://roadmap.powersync.com/

I think currently we post proposals on the Proposals section in GitHub Discussions but I'm not sure if we expect community members to add proposals there. We could mention that Proposals can be found there though

Tagging @kobiebotha

[khawarizmus]

Thanks for the review @cahofmeyr

I really like the way Vue does it. They have a dedicated repo for RFCs. RFC have a lifecycle and a review process. this way anyone can actually propose new feature but only the ones agreed by the core team would make it as an RFC and eventually be realized into a working feature in Vue. Also an approved proposal doesn't mean it can be implemented. some proposals/RFCs where dropped in the past.

In the meantime I think it might be better to have a dedicated section for roadmap and proposals.

[kobiebotha]

It would be great to see some technical proposals submitted by the community.

I'm thinking we can go with these two principles:

• For regular "feature requests", submit/upvote ideas on the roadmap. The user wants a feature and doesn't care how it gets done
• For power users who really want to contribute and help us accelerate progress, they can submit technical proposals/comment on existing proposals

Do you want to take a stab at wording that @khawarizmus ?