SJIP: Clarify the guideline about invariants/properties

[WangSecurity]

Description

Clarify that only the contest README (and the specific question of README) can be used to define the invariants, not the restrictions or expected functionality of the protocol in general.

Judging Guidelines PR

sherlock-protocol/sherlock-v2-docs#58

Rationale

Recently, this specific line confused, and one of the Watsons interpreted this line as only the README can be used to define the protocol's restrictions or expected behaviour. However, it's not correct:

1. This line refers only to guidelines.
2. The restrictions/expected behaviour can be learnt from the code comments, as it's also a source of truth.

Hence, this SJIP is to clarify that this line only refers to invariants/properties of the protocol.

Relevant Issue Discussions

Discussion was on discord.