DOC: Add note that user needs to increase max open files limit if using several tilesets

[brendan-ward]

Observed on MacOS 10.15 with 25 tilesets, it is necessary to raise the max open files limit > 256 (default) or server panics at startup. This may be true for other environments as well. This appears to be due to the switch to crawshaw.io/sqlite from mattn/go-sqlite3, which did not have that issue.