Document compat. guarantees

[tnull]

Besides comments in the code, we'll add a dedicated doc that tracks (in particular forward) compat changes in question and at which point in time we'll consider breaking compat for them. This allows us to clean up our code base when possible.

[tnull]

We should still do that when we encounter the first instance requiring breaking change. Until then it doesn't really make sense to create an empty doc. Removing this from the 0.1 milestone.

[tnull]

Also, we need to investigate what compat guarantees we can reasonably provide for the BDK side of things (BDK 1.0 upgrade will likely be a non-forward compatible change, and I'm not sure BDK's compat. policy after has been decided yet).

[tnull]

Moving this to 0.2 milestone, as only after BDK 1.0 has been finalized we can really be sure what guarantees we can give.