Added CMake command to dev docs for linking against librocksdb

[ejmg]

In the current docs for linking against libsrockdb, building libsnappy.a will fail because of an error thrown while building its third party benchmarks which are enabled by default.

Describe your changes

This PR adds the CMake CLI option to the docs so that snappy's optional benchmarks are disabled by default.

Issue ticket number and link

NA

Checklist before requesting a review

• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes, for the following reason:

This is a one line change to developer docs for linking against a crate dependency.

[conorsch]

Thanks for taking the time, @ejmg! Just-merged is a more comprehensive dev env in #4726, which should do the needful on setting up local tooling, regardless of your workstation platform.