Update v6 migration guide for default strict bytes checking

Author: fselmo

docs/v6_migration.rst

@@ -9,6 +9,22 @@ project depends on Web3.py v6, then you'll probably need to make some changes.
 
 Breaking Changes:
 
+Strict Bytes Checking by Default
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Web3.py v6 moved to requiring strict bytes checking by default. This means that if an
+ABI specifies a ``bytes4`` argument, web3.py will invalidate any entry that is not
+encodable as a bytes type with length of 4. This means only 0x-prefixed hex strings with
+a length of 4 and bytes types with a length of 4 will be considered valid. This removes
+doubt that comes from inferring values and assuming they should be padded.
+
+This behavior was previously available in via the ``w3.enable_strict_bytes_checking()``
+method. This is now, however, a toggleable flag on the ``Web3`` instance via the
+``w3.strict_bytes_type_checking`` property. As outlined above, this property is set to
+``True`` by default but can be toggled on and off via the property's setter
+(e.g. ``w3.strict_bytes_type_checking = False``).
+
+
 Snake Case
 ~~~~~~~~~~