add external access docs (#154)

* add external access docs

* fix typo

docs/reference/external-access.md

@@ -0,0 +1,19 @@
+# External access
+The Jenkins charm requires access to the following domains to install Jenkins and its plugins:
+
+* `jenkins-ci.org`
+* `updates.jenkins-ci.org`
+* `jenkins.io`
+* `updates.jenkins.io`
+* `.mirrors.jenkins-ci.org`
+* `fallback.get.jenkins.io`
+* `get.jenkins.io`
+* `pkg.jenkins.io`
+* `archives.jenkins.io`
+* `pkg.origin.jenkins.io`
+* `.mirrors.jenkins.io`
+* `www.jenkins.io`
+
+Depending on the localisation, some region-specific external mirrors might also be used. You can find more information on the [list of mirrors for Jenkins](https://get.jenkins.io/war/2.456/jenkins.war?mirrorstats).
+
+Some plugins can also require external access, such as `github.com` for the [Github branch source plugin](https://plugins.jenkins.io/github-branch-source/). Or an external Kubernetes cluster if you are using the [Kubernetes plugin](https://plugins.jenkins.io/kubernetes/). Refer to the documentation of the plugin for more details.