docs: add README with instructions

docs/cpp-api/README.md

@@ -0,0 +1,29 @@
+# IfcOpenShell C++ API documentation
+
+This folder contains the setup to build the IfcOpenShell C++ API documentation from the source code.
+
+## Generating the documentation
+
+Prerequisites: make sure to have [Doxygen](https://www.doxygen.nl) and [Graphviz](https://graphviz.org) installed into your `$PATH` variable.
+
+Build with the command (from within the `/docs/cpp-api` folder):
+
+```shell
+$ doxygen
+```
+
+To include the current git commit hash into the build documentation, use the following command:
+
+```shell
+$ PROJECT_NUMBER=$(git rev-parse --short HEAD) doxygen
+```
+
+This will extract the current commit hash in short version and sets the propper ENV variable used by doxygen.
+
+The generation of the documentation might take a while depending on your systems hardware, as it is configured to generate the Class graphs using .
+
+The resulting documentation is located unter `/cpp-api/output/html` and can be directly accessed with your browser:
+
+```shell
+$ open ./output/html/index.html
+```