Update Best Practices with additional transfer types

[isabelle-dr]

google/transit#303 adds two transfer types (4 and 5).
The Best Practices section about transfers.txt should be modified to reflect this change.

[Sergiodero]

As GTFS Best Practices (BP) are currently in the process of being merged to the specification, MobilityData is migrating outstanding issues and PRs from this repository to google/transit. Thus, this issue will be closed and further discussion regarding this BP should take place in google/transit. Please refer to Issue #421 for a more detailed explanation of the migration process and the proposed next steps.

With this, we’re hoping to bring more visibility to outstanding BP issues and to restart the discussion around them, so that any improvement that the community finds valuable could be carried forward.