[Docs] Add README for `openassetio.test` manager test harness

[feltech]

What

Add a README explaining the purpose and usage of the manager test harness.

Why

This follows https://github.com/OpenAssetIO/OpenAssetIO/pull/740#issuecomment-1315638789 (and related earlier comments).

It is not immediately clear when browsing the repository that openassetio.test.manager is not related to our unit tests.

It's also an independent executable that is somewhat distinct from the rest of the openassetio package and could do with it's own documentation.