Guidelines for OPeNDAP directory and file structure

[gryffydd]

When using Thredds/OPeNDAP, there are directory structure issues. If you have too many files in a directory (>1000) you suffer from timeouts when reading the directory and that makes the tool unusable.

[hot007]

I think this goes in the Tech pages somewhere but I'm not sure where. Certainly good advice to keep file counts below 1000/directory though - maybe this could be added to the DRS page in PR #35 ?