Typos in documentation/`README`/`README.*`

[jamesmckinna]

#2176 fixes:

• README: Agda version number should be 2.6.4

See branch documentation where I'm collecting these towards an eventual PR for rc1/rc2...:

• README: Missing comma L10 at "Bradley Hardy Joachim Breitner,"
• Relation.Nullary.Decidable.Core: corrected reference to README.Design.Decidability
• installation-guide (thanks @Saransh-cpp !)
• instance arguments discussion in CHANGELOG
• ...
• documenting the 'how to typecheck README.agda question/problem... UPDATED: now going to be handled in a separate PR by @gallais

Now leaving this issue open, to accumulate any of the other documentation things we might find...?

[Saransh-cpp]

minor thing: installation guide can be updated for v2.0 in the documentation branch (can also be updated for v1.7.3 in master, but v1.7.3 is already out)