Add README for hotfunctions go example and update docs to point to it

examples/tutorial/hotfunctions/http/go/README.md

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+# Hot Function Example
+
+This is an example of a hot function. The [hot function documentation](/docs/hot-functions.md) contains an analysis of how this example works.
+
+### How to run the example
+
+Install the CLI tool, start a Fn server and run `docker login` to login to DockerHub. See the [front page](README.md) for instructions. 
+
+Set your Docker Hub username 
+
+```export FN_REGISTRY=<DOCKERHUB_USERNAME>
+```
+
+Build and deploy the function to the Fn server (default localhost:8080)
+
+fn deploy --app hot-app
+```
+
+Now call your function:
+
+```curl -X POST -d "World" http://localhost:8080/r/hot-app/%2Fhotfn-go
+```