feat: add explanation for testing subgraph extended classes

Description

The PR adds an explanation about how how Aragon uses extended classes to ease the testing for subgraph.

See the framework lifecylce in packages/contracts/docs/framework-lifecycle to decide what kind of change this pull request is.

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

[x] I have selected the correct base branch.
[x] I have performed a self-review of my own code.
[ ] I have commented my code, particularly in hard-to-understand areas.
[ ] My changes generate no new warnings.
[ ] Any dependent changes have been merged and published in downstream modules.
[x] I ran all tests with success and extended them if necessary.
[ ] I have updated the CHANGELOG.md file in the root folder.
[ ] I have updated the DEPLOYMENT_CHECKLIST file in the root folder.
[ ] I have updated the UPDATE_CHECKLIST file in the root folder.
[ ] I have updated the Subgraph and added a QA URL to the description of this PR.