Add reference to OSA repo from main documentation page

In order to make it easy for users to find, let's add OSA reference
to the main page of ONAP documentation

Issue-ID: SECCOM-239
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Change-Id: I2c009a1a21289c2dd18f0e60122a24ec86aefb40
1 file changed