Add instructions to build documentation (#822)

Co-authored-by: Pieter Marsman <[email protected]>

docs/README.md

@@ -0,0 +1,34 @@
+# Working on documentation
+
+The pdfminer.six docs are generated with [Sphinx](https://www.sphinx-doc.org/en/master/), using
+[reStructuredText](https://docutils.sourceforge.io/rst.html).
+
+The documentation is hosted on https://pdfminersix.readthedocs.io/. 
+
+## Deploying new documentation
+
+New documentation is deployed automatically when PR's are merged.
+
+## Building documentation locally
+
+You can build the documentation locally on your machine using the following steps. 
+
+1. (Recommended) create a and activate a Python virtual environment. 
+
+    ```console
+    python -m venv .venv
+    source .venv/bin/activate
+    ```
+   
+2. With the virtual environment activated, install the dependencies for building the documentation. 
+
+    ```console
+    pip install '.[docs]'
+    ```
+   
+3. Build the documentation. 
+
+    ```console
+    make clean && make html
+    ```
+