add install.rst and update index.rst

docs/index.rst

@@ -12,5 +12,6 @@ Documentation Contents
 .. toctree::
    :maxdepth: 2
 
+   install.rst
    api.rst
 

docs/install.rst

@@ -0,0 +1,43 @@
+============
+Installation
+============
+
+1. Install the environment manager :code:`mamba`
+2. Install the Python build tool :code:`poetry`
+
+3. Create an environment with Python, Cython, and Pysam:
+
+.. code-block:: bash
+
+   mamba env create -f pybwa.yml
+
+4. Activate the environment:
+
+.. code-block:: bash
+
+   mamba activate pybwa
+
+5. Clone the :code:`bwa` repo
+
+.. code-block:: bash
+
+   git clone https://github.com/lh3/bwa
+
+6. Configure poetry to install into pre-existing virtual environments:
+
+.. code-block:: bash
+
+    poetry config virtualenvs.create false
+
+7. Install :code:`pybwa` into the virtual environment:
+
+.. code-block:: bash
+
+    poetry install
+
+8. Check your build:
+
+.. code-block:: bash
+
+    poetry run pytest
+