Include reminder on adding 'migration script' in dev guide

[jovatn]

When existing YANG models are modified, a migration script should be added to ensure existing configs are handled properly upon upgrade.
See issue #1454 for an example.

The task for this issue is to include a note in proper location of Developers Guide to make it easier for new developers to do this right.

[troglobit]

Added a Configuration Migration section to the Developer's Guide explaining when and how to write migration scripts, with a pointer to the worked example in src/confd/share/migrate/1.6/ and the user-facing upgrade docs.