Update README.md

Adding introductory paragraph and installation notes.

Author: jbrooke-montala

README.md

@@ -1,2 +1,6 @@
-# docker
-Docker image for ResourceSpace
+# resourcespace/docker
+The official Docker image for ResourceSpace. Full build instructions can be found on our [Knowledge Base](https://www.resourcespace.com/knowledge-base/systemadmin/install_docker).
+
+# Installation notes
+* Before building the Docker image, change the db.env file replacing the default "change-me" passwords to secure values.
+* When setting up ResourceSpace ensure you enter "mariadb" as the MySQL server instead of "localhost" and leave the "MySQL binary path" empty.