Add important notice on experimental branches

[RigoLigoRLC]

This part is written by me voluntarily after discussion on broken models so I felt I should PR this and make this a contribution.

I left two (#xxxx) placeholders for issue links and this should be filled out/removed before merging.

[RigoLigoRLC]

@yqzhishen ping

[yqzhishen]

I think this can be turned into a Compatibility Statements section to point out our rule for maintenance of branches. Key points are:

1. Only the main branch keeps backward compatibility.
2. If there are going to be breaking changes, relevant features will be marked as obsolete or not recommended (in both docs and release notes) for several months in advance.
3. Other branches and forks are not guaranteed to work, including but not limited to: (1) code (or part of the features) may not run, or may not produce correct results; (2) model may not converge, or may not produce reasonable results; (3) old features and models may not behave like they used to; (4) config files and models created on other branches may not work even after merging to main; (5) ONNX models exported on other branches may not work (and may never work in the future) in the main branch of downstream infrastructures like OpenUTAU, or may not produce correct results even if they seem to be sucessfully exported.
4. Include a discouraging statement to prevent users to try other branches for production use or showing off in public, unless we post special issues or discussions for public beta involving other branches.
5. Maybe there can be some explanations for why adding these statements?