docs(doc): add README with the instructions on how to build the html …

…documentation

doc/README.md

@@ -0,0 +1,21 @@
+# HTML documentation
+
+QUEENS uses [Sphinx](https://www.sphinx-doc.org/en/master/) to automatically build an html-documentation from the docstrings.
+
+## Build the documentation
+
+When building the documentation on your machine for the first time or after adding new modules or classes to QUEENS, one needs to first rebuild the `autodoc index` by running:
+
+```bash
+cd <queens-base-directory>
+sphinx-apidoc -o doc/source queens -f -M
+```
+
+To actually build the html-documentation, navigate into the doc folder and run the make command:
+
+```bash
+cd doc
+sphinx-build -b html -d build/doctrees source build/html
+```
+
+You can now view the documentation in your favorite browser by opening `build/html/index.html`