infiniband: add a Documentation driver-api chapter for Infiniband

Add a driver-api chapter for InfiniBand interfaces.

Link: https://lore.kernel.org/r/20191010035240.310347906@gmail.com
Signed-off-by: Randy Dunlap <rd.dunlab@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 38e638a..d0efa35 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -26,6 +26,7 @@
    device_link
    component
    message-based
+   infiniband
    sound
    frame-buffer
    regulator