Mention the default location for temporary files

README.md

@@ -69,6 +69,7 @@ Options
  * -F or --allow-existing: Proceed (without deleting existing data) if the metadata or tiles table already exists
    or if metadata fields can't be set
  * -t _directory_ or --temporary-directory=_directory_: Put the temporary files in _directory_.
+   If you don't specify, it will use `/tmp`.
  * -P or --read-parallel: Use multiple threads to read different parts of each input file at once.
    This will only work if the input is line-delimited JSON with each Feature on its
    own line, because it knows nothing of the top-level structure around the Features. Spurious "EOF" error