Add troubleshooting section for https (#754)

* Add troubleshooting section for https

Co-Authored-By: Luke Kysow <1034429+lkysow@users.noreply.github.com>

runatlantis.io/docs/troubleshooting-https.md

@@ -0,0 +1,27 @@
+# HTTPS, SSL, TLS
+
+When using a self-signed certificate for Atlantis (with flags `--ssl-cert-file` and `--ssl-key-file`),
+there are a few considerations.
+
+Atlantis uses the web server from the standard Go library, 
+the method name is [ListenAndServeTLS](https://golang.org/pkg/net/http/#ListenAndServeTLS).
+
+`ListenAndServeTLS` acts identically to [ListenAndServe](https://golang.org/pkg/net/http/#ListenAndServe),
+except that it expects HTTPS connections. 
+Additionally, files containing a certificate and matching private key for the server must be provided. 
+If the certificate is signed by a certificate authority, 
+the file passed to `--ssl-cert-file` should be the concatenation of the server's certificate, any intermediates, and the CA's certificate. 
+
+If you have this error when specifying a TLS cert with a key: 
+```
+[EROR] server.go:413 server: Tls: private key does not match public key
+```
+
+Check that the locally signed certificate authority is prepended to the self signed certificate.
+A good example is shown at [Seth Vargo terraform implementation of atlantis-on-gke](https://github.com/sethvargo/atlantis-on-gke/blob/master/terraform/tls.tf#L64)
+
+For Go specific TLS resources have a look at the repository by [denji called golang-tls](https://github.com/denji/golang-tls).
+
+For a complete explanation on PKI, read this [article](https://smallstep.com/blog/everything-pki.html).
+
+