Update docs to mention including ast

[gasperbr]

With a recent update the ast has been removed from the default forge build output which causes scripts to fail with a no matching value found at ".ast.absolutePath" message.

[ericglau]

Hi @gasperbr, thanks for the PR. Which version of forge are you running, can you share the output of forge --version ?

[ericglau]

I see the issue if running with the default foundry.toml, e.g. without any of the following:

build_info = true
ast = true
extra_output = ["storageLayout"]

It looks like the error is occuring here: https://github.com/OpenZeppelin/openzeppelin-foundry-upgrades/blob/v0.1.0/src/internal/Utils.sol#L83

I think ideally a check should be added before this line, and if the JSON does not contain the expected ast key, a helpful error should be thrown to direct users to edit their foundry.toml as above.

Would you be interested in adding such a check? No worries if not, I could also add it. Thanks.

[ericglau]

Updated docs and added the check mentioned in my comment above.