Need to document spack and ramble version pinning

[pearce8]

This can come up when adding or modifying a benchmark - and the package.py is already in Spack, but not yet in Benchpark.

• how to find the versions we are on
• potential side effects (e.g., version compatibility issues (e.g., package in tip of develop vs. our pinned version))
• what might happen if people manually get the tip of develop (and how they could do that if they really needed to)