Add a note for contributors installing fourmolu

- Code review suggested changes
  - Redundant "that".
  - Use indefinite article.
  - Assume ghc version installed via ghcup
  - Use `--with-compiler` option
  - Grammar, "its requirement"
- Move --with-compiler to a tip
- Use --numeric-version
- Use hyphen in series-specific
- Capitalise GHC
- Note restriction on system ghc version.
- Add a warning about switching fourmolu versions

Co-Authored-By: brandon s allbery kf8nh <[email protected]>
Co-Authored-By: Artem Pelenitsyn <[email protected]>

CONTRIBUTING.md

@@ -190,15 +190,55 @@ of their patch is not influenced by their setup or implicit knowledge of the sys
 
 ## Code Style
 
-We use automated formatting with Fourmolu to enforce a unified style across the code bases. It is checked in the CI process.
-After installing Fourmolu 0.12, there are some makefile targets to help formatting
-the code base.
-
+We use automated formatting, checked in the CI process, to enforce a unified
+style across the code bases and include these makefile targets to help with
+formatting:
 
 * `make style` - Format the `Cabal`, `Cabal-syntax` and `cabal-install` directories.
 * `make style-modified` - Format files modified in the current tree.
 * `make style-commit COMMIT=<ref>` - Format files modified between HEAD and the given reference.
 
+> [!NOTE]
+> We use `fourmolu-0.12.0.0` for formatting. If installing it with `cabal
+> install`, please make sure to use a version of `GHC >= 9.2.1 && < 9.8`.
+> Its requirement of `GHC2021` sets the lower bound, and its reliance on
+> `ghc-lib-parser` sets the upper bound on GHC versions. A command for
+> installing it this way is:
+>
+> ```
+> $ cabal install fourmolu-0.12.0.0 --overwrite-policy=always --ignore-project
+> ```
+
+> [!Tip]
+> If you have multiple versions of GHC installed with `ghcup`, a series-specific
+> GHC can be used by adding an option of `--with-compiler=ghc-x.y` (for the
+> latest installed `x.y.z` version in the `x.y` series) or an option of
+> `--with-compiler=ghc-x.y.z` to the above install command.
+>
+> ```
+> $ ghc --numeric-version
+> 9.10.1
+>
+> $ ghc-9.6 --numeric-version
+> 9.6.6
+>
+> $ ghc-9.4.8 --numeric-version
+> 9.4.8
+> ```
+>
+> If not using the `--with-compiler` option then the system `ghc` version should
+> be one in the `ghc-9.2`, `ghc-9.4` or `ghc-9.6` series when installing
+> `fourmolu`.
+
+> [!WARNING]
+> If you have need of another `fourmolu` version for other work and want to
+> switch between versions then your options are reinstalling or using a nix shell
+> or something similar. It is not yet possible to specify development
+> environment tools within a package with different constraints on dependencies,
+> see [issue-9230][issue-9230].
+
+[issue-9230]: https://github.com/haskell/cabal/issues/9230
+
 ## Whitespace Conventions
 
 We use automated whitespace convention checking. Violations can be fixed by