docs/interfaces.txt: add documentation to describe interface differences

Provide a simple documentation that defines the main userspace
verbs interfaces and the differences between them.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
git-svn-id: svn://openib.tc.cornell.edu/gen1@2673 ad392aa1-c5ef-ae45-8dd8-e69d62a5ef86

diff --git a/trunk/docs/interfaces.txt b/trunk/docs/interfaces.txt
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+WinOF provides several userspace interfaces.  The main application\r
+interfaces are given below, along with a brief description of each.\r
+\r
+\r
+libibverbs\r
+OFA interface available on windows and linux\r
+       This is the only linux verbs interface, which was ported to\r
+       windows to support portability.  This sits over winverbs.\r
+       It provides low abstraction to the hardware, but does not\r
+       expose windows specific features, like overlapped operations\r
+       for asynchronous control.  This is a good choice for low-level\r
+       access, with portability between windows and linux.\r
+\r
+WinVerbs\r
+OFA windows interface\r
+       Lowest level windows verbs interface for applications.  Most\r
+       of the other interfaces sit over this interface.  This provides\r
+       the lowest abstraction to the hardware, while also exposing\r
+       windows specific features.  This is a good choice for\r
+       low-level access on windows.\r
+\r
+DAPL (Direct Access Programming Library)\r
+Industry standard RDMA interface for multiple OS's\r
+       Hardware providers may support this interface, even if not\r
+       part of OFA.  This sits over libibverbs and ibal.  This provides a\r
+       higher level of abstraction, but provides the most portability.\r
+\r
+Network Direct (ND)\r
+Microsoft defined interface for windows\r
+       Hardware providers may support this interface, even if not\r
+       part of OFA.  This sits over winverbs and ibal.  This provides\r
+       a higher level of abstraction, but may provide access to the\r
+       most hardware on Windows platforms.\r
+\r
+IBAL (InfiniBand Access Layer)\r
+OFA windows interface\r
+       Older windows interface.  This is currently a peer to winverbs,\r
+       with a longer term goal of moving it over winverbs.  The\r
+       interface will be supported going forward.  This provides low\r
+       level abstraction, but does not expose windows specific features.\r