doc: man-page for source command

Provide a man-page for the source command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index cf3666a..53e4aee 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -75,6 +75,7 @@
    cmd/setexpr
    cmd/size
    cmd/sound
+   cmd/source
    cmd/temperature
    cmd/tftpput
    cmd/true