Documentation notes about running Fn on SELinux systems (#507)

Documentation notes about running Fn on SELinux systems

docs/operating/selinux.md

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+# Running on SELinux systems
+
+Systems such as OEL 7.x where SELinux is enabled and the security policies are set to "Enforcing" will restrict Fn from
+running containers and mounting volumes.
+
+For local development, you can relax SELinux constraints by running this command in a root shell:
+
+```sh
+setenforce permissive
+```
+
+Then you will be able to run `fn start` as normal.
+
+Alternatively, use the docker-in-docker deployment that a production system would use:
+
+```sh
+docker run --privileged --rm --name fns -it -v $PWD/data:/app/data -p 8080:8080 fnproject/functions
+```
+
+Check the [operating options](options.md) for further details about this.