Add a note about providing stable interfaces

[arunkannawadi]

upon transferring code between packages.

Recently during a conversation with @TallJimbo , we discussed how Science Pipelines is intended to be used as a (stable enough) library by third-party but how we actually don't provide any such stability in practice. In an ongoing code transfer ticket, I was reminded of this and think it'd be good to explicitly include this in the instructions for transferring code between packages.

[arunkannawadi]

Build webpage is here: https://developer.lsst.io/v/u-arunkannawadi-importability-on-code-transfer/stack/transferring-code.html