Add a known issue about ImplicitCAD in cabal.projects packages

Adding implicit (and possibly linear) causes hint to fail interpreting files using these.

E.g. (in `cabal.project.local`)
```
packages:
../ImplicitCAD/implicit.cabal
```

Probably worth mentioning this footgun somewhere.

Start by reproducing the failure with the `cabal.project.local` example that adds `../ImplicitCAD/implicit.cabal`, and check how `hint` handles files using ImplicitCAD and possibly `linear`. Find the appropriate project documentation location and record this dependency-related footgun as a known issue.

索引モデルが issue の本文から書いたものです。