Added script integration test intructions to developer-notes

Summary: Working on D1381 made it clear that these instructions are necessary, as they aren't documented anywhere.

Test Plan: D1381 shows these instructions are correct

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1411