ADR 006: Upgrade client implementation proposal (#383)

* Initial ADR-006 draft

* Apply some revisions

* Add a description for zero_custom_fields

* Apply some revisions

* Apply review comments

* Remove trusting period bound for general MUSTs

* Revise clause related to proof verification

* Add a NOTE for chain-level upgrade steps

* Reword height check clause

* Add description for sentinel value

* Clarify basic vs upgrade-specific validations + notice for upgradedConsState

Author: Farhad-Shabani