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

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.