add server installation notes

Author: hectorsector

server-installation-notes.md

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+## GitHub Enterprise Server (GHES) installation notes
+
+🚫 | This course needs features not yet supported by GHES.
+--- | ---
+
+This course makes use of [GitHub Actions](https://github.com/features/actions), a feature that is currently available for github.com but not available on GHES.
+
+### Course dependencies
+
+The following are dependencies of the course. The course may continue to work without these dependencies, but learners won't experience the course as designed.
+
+| Dependency                                                                                                                      | Required? | Reason                                                                                                                                           | Alternative                                                                                                                                                                                    |
+|---------------------------------------------------------------------------------------------------------------------------------|-----------|--------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| GHES must be able to reach githubusercontent.com                                                                                | Yes       | Images used throughout the course are served from this domain. Learners will find broken images required to take the course without this access. | Manually download the images referenced in the `responses/` folder, upload them to an accessible domain, and replace the images in the `responses/` folder.                                    |
+| Learner must be able to reach github.com and outside web                                                                        | No        | Links are provided to resources that live on the outside web.                                                                                    | Without access to resources on the outside web, learners will reach blocked resources. You can change the links to these resources in the `responses/` folder, and in the template repository. |