CONTRIBUTING.md: be more explicit about some of our packaging reqts

[classabbyamp]

this is stuff we've said to many contributors but haven't really written down

[icp1994]

This might be off-topic for the proposed changes, but I would put emphasis somewhere that it's better to install & update packages in the user-space via language specific tools (cargo, uv etc) if possible rather than installing them system-wide via xbps. This primarily applies to self-contained "apps" which do not need global installations. Also helps the user/maintainer having to worry about conflicting dependencies with existing ones in some cases - for example python packages.