Init commit of 3d party access documentation.

x-road/3rd-party-access-to-bbs.md

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+# Third party access to the building blocks
+
+![USCT demo GovStack instance](./assets/usct-govstack-instance.drawio.png)
+
+
+On demand, sandbox team can up and run security server to access the sandbox.
+
+The security server API will be firewall protected (IP whitelisting) and the UI will be credential protected.
+
+The security server will be pre-configured, but client can change configuration and play with it. 
+
+Dedicated security server that is connected to X-road and has the right to communicate with other building blocks.
+
+## Provision guide line
+1. Receive IP address from the client.
+2. Deploy new security server to the playground cluster.
+3. Configure new security server and others for communication.
+4. Configure firewall rules 
+5. Generate user interface credentials.