added iosystem.txt

doc/source/Makefile.am

@@ -5,4 +5,4 @@
 EXTRA_DIST = api.txt CAMexample.txt Decomp.txt faq.txt Installing.txt	\
 Testing.txt base.txt c_api.txt contributing_code.txt Error.txt		\
 Examples.txt Introduction.txt mach_walkthrough.txt			\
-testpio_example.txt users_guide.txt
+testpio_example.txt users_guide.txt iosystem.txt

doc/source/iosystem.txt

@@ -0,0 +1,24 @@
+/** @page iosystem Initializing the IO System
+
+Using PIO begins with initializing the IO System. This sets up the MPI
+communicators with the computational and I/O processors.
+
+When the IO System is created, an IOSystem ID is returned and must be
+used in future PIO calls. The IOSystem ID is returned by C functions
+PIOc_Init_Intracomm() and PIOc_init_async(). Fortran users see @ref
+PIO_init.
+
+When the user program is complete, the IOSystem should be released by
+calling C function PIOc_finalize() or Fortran function piolib_mod::finalize()
+for each open IOSystem.
+
+@section intercomm_mode Intercomm Mode
+
+@image html PIO_Intercomm1.png "PIO Intercomm Mode"
+
+@section async_mode Async Mode
+
+@image html PIO_Intracomm1.png "PIO Async Mode"
+
+*/
+

doc/source/users_guide.txt

@@ -8,6 +8,7 @@ examples on how it can be used. Please watch the PIO GitHub site
 releases.
 
  - @ref intro
+ - @ref iosystem
  - @ref decomp
  - @ref error
  - @ref test