Update README

README.md

-## Search Doc Service 
+## Search Doc Service
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+
+A service which accepts memobase JSON-LD records, record sets and institutions and transforms them into their
+respective index format. 
+
+This is a highly specific transformation with many extra tweaks specifically needed by the frontend search.
+
+[Confluence Doku](https://ub-basel.atlassian.net/wiki/spaces/MEMOBASE/pages/602210371/Search+Index+Transformation)