Make parallel reading a command-line option

man/tippecanoe.1

@@ -5,6 +5,9 @@ Builds vector tilesets
 \[la]http://geojson.org/\[ra]
 features. This is a tool for making maps from huge datasets
 \[la]MADE_WITH.md\[ra]\&.
+.PP
+[Build Status](https://travis\-ci.org/mapbox/tippecanoe.svg)
+\[la]https://travis-ci.org/mapbox/tippecanoe\[ra]
 .SH Intent
 .PP
 The goal of Tippecanoe is to enable making a scale\-independent view of your data,
@@ -67,6 +70,12 @@ specified, the files are all merged into the single named layer.
 \-f: Delete the mbtiles file if it already exists instead of giving an error
 .IP \(bu 2
 \-t \fIdirectory\fP: Put the temporary files in \fIdirectory\fP\&.
+.IP \(bu 2
+\-P: 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.
+In addition, it only works if the input is a named file that can be mapped into memory
+rather than a stream that can only be read sequentially.
 .RE
 .SS Zoom levels and resolution
 .RS