Add documentation about the github actions (#942)

* Add documentation on the github-actions

* Rename CI.md to README.md

* Formatting

* Add conda-package-publish-action reference

* Finish sentence about previewing docs

* Specify that annotated tagging is the action trigger

* Add notes from docs action PR

#880

* Formatting

* Specify linux build

* Fix links so they are to master, not other branches

.github/workflows/README.md

@@ -0,0 +1,52 @@
+# GitHub Actions
+
+## Building the Conda Package: [conda_build](https://github.com/TomographicImaging/CIL/blob/master/.github/workflows/conda_build.yml)
+This github action builds and tests the conda package, by using the [conda-package-publish-action](https://github.com/paskino/conda-package-publish-action)
+
+When pushing to master or creating an [annotated tag](https://git-scm.com/book/en/v2/Git-Basics-Tagging), *all* variants are built and tested.
+
+When opening or modifying a pull request to master, a single variant is built and tested. This variant is for linux with `python=3.7` and `numpy=1.18`.
+
+The action does not publish to conda, instead this is done by jenkins. This is because github-actions do not have a GPU.
+
+## Building/Publishing Documentation: [docs_build_and_publish](https://github.com/TomographicImaging/CIL/blob/master/.github/workflows/docs_build_and_publish.yml)
+
+This github action builds and optionally publishes the documentation located in [docs/source](https://github.com/TomographicImaging/CIL/tree/master/docs/source). 
+
+The github action has two jobs:
+1. [build](https://github.com/TomographicImaging/CIL/blob/master/.github/workflows/docs_build_and_publish.yml#L12): 
+-  builds the documentation with sphinx
+-  uses upload-artifact to upload the html files which may then be used by **publish**
+
+2. [publish](https://github.com/TomographicImaging/CIL/blob/master/.github/workflows/docs_build_and_publish.yml#L40):
+-  uses download-artifact to retrieve the built html files
+-  pushes the html files to the `nightly` folder on the gh-pages branch
+
+If opening or modifying a pull request to master, `build` is run, but not `publish`.
+If pushing to master or tagging, the documentation is built *and* published (both the `build` and `publish` jobs are run).
+
+### Viewing Built Documentation
+The `build` job builds the documentation and uploads it as an [artifact](https://github.com/TomographicImaging/CIL/blob/master/.github/workflows/docs_build_and_publish.yml#L36),
+in a folder named `DocumentationHTML`.
+This can be found by going to the ‘Actions’ tab, and selecting the appropriate run of `.github/workflows/docs_build_and_publish.yml`.
+
+When viewing the summary for the run of the action, there is an `Artifact` section at the bottom of the page.
+Clicking on `DocumentationHTML` allows you to download a zip folder containing the built html files. This allows you to preview the documentation site before it is published.
+
+### Publication of the Documentation
+The documentation is hosted on the [github site](https://tomographicimaging.github.io/CIL/) associated with the repository.
+This is built from the [gh-pages branch](https://github.com/TomographicImaging/CIL/tree/gh-pages). 
+
+If you are an admin of the CIL repository you are able to see the settings for the site by going to `Settings->Pages`.
+
+To publish the documentation, the publish job of the gh-action pushes the documentation changes to the `gh-pages` branch.
+Any push to this branch automatically updates the github site.
+
+
+### Initial Setup of the Docs Site & Action
+To get the action to work I first had to:
+1. [Create a gh-pages branch](https://gist.github.com/ramnathv/2227408) - note this only worked in bash, not windows command line.
+2. [Set the source](https://github.com/TomographicImaging/CIL/settings/pages) for our github pages to be the gh-pages branch.
+
+I followed the examples on the [sphinx build action page](https://github.com/marketplace/actions/sphinx-build), specifically this [example workflow](https://github.com/ammaraskar/sphinx-action-test/blob/master/.github/workflows/default.yml)
+