Added some notes on API documentation.

docs/README-API-documentation.txt

@@ -0,0 +1,18 @@
+The API documentation is readable in a few ways:
+
+- Read physfs.h; it's _heavily_ documented and the primary source of reference
+  documentation for the library.
+- Run Doxygen over the header, which produces nicer-to-browse documentation in
+  HTML, LaTeX, manpage, etc formats. This is done for you if Doxygen is
+  installed and you build the "docs" target in whatever project files CMake
+  generated for you.
+- Too much trouble? We generated the HTML reference for you, online here:
+
+      https://icculus.org/physfs/docs/
+
+- We would love well-written tutorials for the latest version of PhysicsFS!
+  If you write one, we would love to list it here. Drop me a line about it:
+  icculus@icculus.org ... Thanks!
+
+--ryan.
+