Use develop as main working branch

[martriay]

Although our docs and official messaging insist in using a version tag when installing the library, some people (maybe many) don't.

[dependencies]
openzeppelin = { git = "https://github.com/OpenZeppelin/cairo-contracts.git", tag = "v0.8.1" }

To be fair, it's ver easy not to. I therefore suggest we start pairing main to the latest release and switch to develop as our working branch in the spirit of making our stuff as hard to mess up with as possible. The change needs to be documented in our CONTRIBUTING guide.