Created HTTP targets on separate domains (markdown)

HTTP-targets-on-separate-domains.md

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+> This feature is available in v0.5+
+
+Instead of using `?warpgate-target=` in the URL, you can use multiple domains/hostnames and link each to a specific target. 
+
+Accessing Warpgate over HTTP on a specific domain will then automatically select the corresponding target.
+
+For optimal results, you want to host Warpgate on a common higher-level domain (e.g. `wg.acme.inc`, as set by the `external_host` config option), with target-specific domains as subdomains of this one (e.g. `gitlab.wg.acme.inc`) - this will prevent users from having to log in again when switching between domains (Warpgate will set its session cookie for all subdomains).
+
+# Linking a target to a domain
+
+Set the `http.external_host` property on the target config:
+
+```diff
+  - name: gitlab
+    allow_roles:
+      - "warpgate:admin"
+      - qa
+    http:
+      url: http://10.0.0.2
++     external_host: gitlab.wg.acme.inc
+```