Update README with chrono_io.

README.md

@@ -10,11 +10,13 @@ This is actually several separate C++11/C++14 libraries:
 
 2. `"tz.h"` / `"tz.cpp"`  are a timezone library built on top of the `"date.h"` library.  This timezone library is a complete parser of the IANA timezone database.  It provides for an easy way to access all of the data in this database, using the types from `"date.h"` and `<chrono>`.  The IANA database also includes data on leap seconds, and this library provides utilities to compute with that information as well.  See http://howardhinnant.github.io/date/tz.html for more details.
 
-3. `"iso_week.h"` is a header-only library built on top of the `"date.h"` library which implements the ISO week date calendar.  See http://howardhinnant.github.io/date/iso_week.html for more details.
+3.  `"chrono_io.h"` is a header-only library for streaming out chrono durations.  See http://howardhinnant.github.io/date/chrono_io.html for more details.
 
-4. `"julian.h"` is a header-only library built on top of the `"date.h"` library which implements a proleptic Julian calendar which is fully interoperable with everything above.  See http://howardhinnant.github.io/date/julian.html for more details.
+4. `"iso_week.h"` is a header-only library built on top of the `"date.h"` library which implements the ISO week date calendar.  See http://howardhinnant.github.io/date/iso_week.html for more details.
 
-5. `"islamic.h"` is a header-only library built on top of the `"date.h"` library which implements a proleptic Islamic calendar which is fully interoperable with everything above.  See http://howardhinnant.github.io/date/islamic.html for more details.
+5. `"julian.h"` is a header-only library built on top of the `"date.h"` library which implements a proleptic Julian calendar which is fully interoperable with everything above.  See http://howardhinnant.github.io/date/julian.html for more details.
+
+6. `"islamic.h"` is a header-only library built on top of the `"date.h"` library which implements a proleptic Islamic calendar which is fully interoperable with everything above.  See http://howardhinnant.github.io/date/islamic.html for more details.
 
 There has been a recent change in the library design.  If you are trying to migrate from the previous design, rename `day_point` to `sys_days` everywhere, and that ought to bring the number of errors down to a small roar.