add installation notes

Author: hectorsector

server-installation-notes.md

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+# GitHub Enterprise Server (GHES) installation notes
+
+✅ | This course is fully compatible with any supported version of GHES.
+--- | ---
+
+Supported versions of GHES are visible by using the drop-down in the [official documentation](https://help.github.com/enterprise/).
+
+## Course dependencies
+
+Dependency | Reason | Alternative
+--- | --- | ---
+[GitHub Pages](https://help.github.com/en/enterprise/2.16/admin/installation/configuring-github-pages-on-your-appliance) | Used to let the learner publish a web site with their own innersource resources. | Without GitHub Pages enabled, the learner can still take the course, but won't be able to publish these resources on their own web site. If pages isn't enabled, you should remove references to a published GitHub Pages site in the `responses/` folder.