Missing guidance on topfinding parallel

Hi `parallel` maintainers,

Having followed the installation directions for oxcaml, I've got a fresh switch with `parallel.v0.18~preview.130.55+197` installed. However, I'm not sure how to bring `parallel` into the toplevel; using a `#require` directive reports the following error:

```
utop # #require "parallel";;
Error: Reference to undefined compilation unit Config
Hint: This means that the interface of a module is loaded, but its implementation is not.
Did you mean to load a compiled implementation of the module
using #load or by passing it as an argument to the toplevel?
─( 14:43:27 )─< command 3 >──────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop #
```

I'm not fully sure how to proceed. This is the only version that seems to be available in opam, so I'm not sure how to try an earlier version:

```
[ntaylor@smuggo ~]$ opam show parallel

<><> parallel: information on all versions ><><><><><><><><><><><><><><><><><><>
name parallel
all-installed-versions v0.18~preview.130.55+197 [5.2.0+ox]
all-versions v0.18~preview.130.55+197
```

A section about how to bring parallel into the top level in the documentation, if I've missed a step, would be appreciated.

Thanks,
Nathan

Start by reproducing the reported `#require "parallel"` failure in utop and review the installation and toplevel context described in the issue. Update the project documentation with guidance for bringing parallel into the toplevel, including any relevant version or loading steps; done means a fresh opam installation has clear instructions.

Written by the indexing model from the issue text.