Updating the readme

Signed-off-by: Mary Anthony <mary@blockstack.com>

README.md

@@ -82,11 +82,19 @@ As of 8/12/19 Clarity is in the [develop](https://github.com/blockstack/blocksta
 2. Build the lastest JSON.
 
    ```
-   $ docker run --name docsbuild -it blockstack-test blockstack-core docgen | jsonpp > ~/repos/docs.blockstack/_data/clarityRef.json
+   docker run -it blockstack/blockstack-core:clarity-developer-preview blockstack-core docgen | jsonpp > ~/repos/docs.blockstack/_data/clarityRef.json
    ```
 
 3. Build the documentation and verify the Clarity reference is building correctly.
 
+4. Make changes in core
+5. Build the docker image
+6. Run doc gen with the new image
+
+```
+   $ docker run --name docsbuild -it blockstack-test blockstack-core docgen | jsonpp > ~/repos/docs.blockstack/_data/clarityRef.json
+```
+
 
 # Technology Reference