Add README with instructions on how to build the documentation.

Signed-off-by: Camila <hello@camila.codes>

doc/README.md

@@ -0,0 +1,17 @@
+# Desktop client documentation
+
+- The main nextcloud Documentation is found at https://github.com/nextcloud/documentation
+- The rst files from the current stable branch will be parsed with sphinx to be used at https://docs.nextcloud.com/desktop/3.0/
+
+## How to build the documentation
+
+- After installing [sphinx](https://www.sphinx-doc.org) you can run:
+
+```
+$ git clone https://github.com/nextcloud/desktop.git
+$ cd desktop
+$ cd docs
+$ sphinx-build -b html -D html_theme='nextcloud_com' -d _build/doctrees   . _build/html/com
+```
+
+The documentation html files will be at ```desktop/docs/_build/html/com```.