Add x509 documentation

Document "x509: certificate signed by unknown authority" issues.

Closes #3613 and #3721

docs/http_proxy.md

@@ -65,6 +65,18 @@ Get https://k8s.gcr.io/v2/: net/http: request canceled while waiting for connect
 
 This error indicates that the container runtime running within the VM does not have access to the internet. Verify that you are passing the appropriate value to `--docker-env HTTPS_PROXY`.
 
+## x509: certificate signed by unknown authority
+
+```
+[ERROR ImagePull]: failed to pull image k8s.gcr.io/kube-apiserver:v1.13.3: output: Error response from daemon: Get https://k8s.gcr.io/v2/: x509: certificate signed by unknown authority
+```
+
+This is because minikube VM is stuck behind a proxy that rewrites HTTPS responses to contain it's own certificate. The [solution](https://github.com/kubernetes/minikube/issues/3613#issuecomment-461034222) is to install the proxy certificate into a location that is copied to the VM at startup. Ask your IT department for the appropriate PEM file, and add it to:
+
+`~/.minikube/files/etc/ssl/certs`
+
+Then run `minikube delete` and `minikube start`.
+
 
 ## Additional Information