Added a README.md as a docsify index of slides

Added a README.md file as a docsify page in the slides directory.
In this way we can create an index of available slides from within the
docsify site for easier navigation.

To make these links work we need to add the :ignore attribute in the
markdown link’s title as per
docsifyjs/docsify#203

slides/README.md

@@ -0,0 +1,5 @@
+# Slides
+
+Hier vind je een overzicht van alle slides die bij deze cursus horen:
+
+- [01.03 Source control](/slides/01.03.source-control/index.html ":ignore")