Add buil instructions

Author: bbayles

docs/index.rst

@@ -1,5 +1,50 @@
 .. include:: ./_build/README.pprst
 
+Building from source
+====================
+
+You will need to have Python 3 development files installed.
+On macOS, you will have these if you installed Python with ``brew``.
+On Linux, you may need to install some packages (e.g., ``python3-dev`` and ``python3-venv``).
+
+You will also need a C++ toolchain.
+On macOS, Xcode will provide this for you.
+On Linux, you may need to install some more pacakges (e.g. ``build-esential``).
+
+Clone the git repository to a directory for development:
+
+.. code-block:: sh
+
+    git clone https://github.com/ada-url/python/ ada_url_python
+    cd ada_url_python
+
+Create a virtual environment to use for building:
+
+.. code-block:: sh
+
+    python3 -m venv env
+    source ./env/bin/activate
+
+After that, you're ready to build the package:
+
+.. code-block:: sh
+
+    python -m pip install -r requirements/development.txt
+    c++ -c "ada_url/ada.cpp" -fPIC -std="c++17" -O2 -o "ada_url/ada.o"
+    python -m build --no-isolation
+
+This will create a `.whl` file in the `dist` directory. You can install it in other
+virtual environments on the same machine.
+
+To run tests, first build a package. Then:
+
+ .. code-block:: sh
+
+    python -m pip install -e .
+    python -m unittest
+
+Leave the virtual environment with the ``deactivate`` comamnd.
+
 API Documentation
 =================