Use ABAP CDS Syntax Highlighter for .acds files

With github-linguist/linguist#4614 Github understands "asddls" files and uses
the syntax coloring provided by
https://github.com/FreHu/abap-cds-grammar

Let's use an override for this repo so that acds files just use the
existing coloring for "asddls".
This is explained in:
https://github.com/github/linguist/blob/master/docs/overrides.md

Fixes: #371

.gitattributes

@@ -0,0 +1 @@
+*.acds linguist-language=ABAP CDS