Add upgrade guide to docs

[ikonst]

PynamoDB 2 -> 3

• https://pynamodb.readthedocs.io/en/stable/upgrading_unicodeset.html

PynamoDB 3 -> 4

• Rewriting conditional expressions (e.g. foo__none=True -> condition=MyModel.foo.does_not_exist())
• Rewriting actions

PynamoDB 4 -> 5

• TODO: Release notes should be split into breaking changes and features
• Maps in lists now have nullability enforced
• Empty strings no longer coalesced to None on read-back

PynamoDB 5 -> 6

• https://pynamodb.readthedocs.io/en/latest/upgrading_binary.html

[zhang-alvin]

not related to this issue directly, but what is the intended release schedule for version 6.0?