update installation docs

python/doc/installation.rst

@@ -1,4 +1,29 @@
 Installation
 ============
 
-TODO. See CASMcode_crystallography/python/doc/installation.rst for similar instructions.
+
+Install from PyPI
+-----------------
+
+.. note::
+
+    CASM is built for macOS x86_64 (Intel), macOS arm64 (Apple Silicon), Linux x86_64, and Linux aarch64.
+
+The latest release of libcasm-clexulator can be installed with:
+
+    pip install libcasm-clexulator
+
+
+Install from source
+-------------------
+
+See the `libcasm contribution guide`_.
+
+
+For contributors
+----------------
+
+See the `libcasm contribution guide`_.
+
+
+.. _`libcasm contribution guide`: https://prisms-center.github.io/CASMcode_docs/pages/contributing_to_libcasm_packages/