cta-dcache: add README.md to decribe the module

cta-dcache/README.md

+# dCache CTA integration
+
+gRPC based frontend for integration with dCache 7.2.2 and later. This is thin layer
+that only populates CTA catalog and scheduler queue. THe connection between a CTA and
+a dCache instance maintained by hsm URI, for example:
+
+```
+osm://cta/00001D43C0C086CA459298C634D67F68AB6B?archiveid=8402
+```
+
+where _00001D43C0C086CA459298C634D67F68AB6B_ corresponds to the dCache unique file id
+(so called pnfs id) and _8402_ corresponds to the archive id within CTA catalog.
+
+> NOTE: the original file name, owner and owner group are not propagated by dCache to CTA.
+
+## Configuration
+
+As mentioned above, the _cta-dcache_ interface requires only connection to the CTA catalog
+and the scheduler. Those configurations are taken from `/etc/cta/cta-catalogue.conf` and
+`/etc/cta/cta.conf`. The TPC port number used to accept dCache requests is specified in
+`/etc/sysconfig/cta-dcache` and defaults to `17017`.
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