Add header to readme link table (#1848)

Having the source and repo link as a 'normal' row make it clearer what
is going on and stops 'Source' looking like a description of the other
link types.

Co-authored-by: oliwenmandiamond <136330507+oliwenmandiamond@users.noreply.github.com>

Author: tpoliaw

README.md

@@ -7,8 +7,9 @@
 
 Ophyd devices and other utils that could be used across DLS beamlines
 
-Source          | <https://github.com/DiamondLightSource/dodal>
+What            | Where
 :---:           | :---:
+Source          | <https://github.com/DiamondLightSource/dodal>
 PyPI            | `pip install dls-dodal`
 Documentation   | <https://diamondlightsource.github.io/dodal>
 Releases        | <https://github.com/DiamondLightSource/dodal/releases>