Add "roadmap" page (#156)

* Add "roadmap" page

* Moving roadmap to contributor page

Author: sunqm

source/contributor/api-v3.rst

@@ -0,0 +1,30 @@
+.. _api_v3:
+
+# PySCF v3 API proposals
+
+.. This page lists proposed changes to the API in PySCF-v3
+
+## Removing TaggedNPArray
+
+### Passing additional keyword arguments
+
+### Returning multiple values instead of the TaggedNPArray
+
+### Global configuration to control the return style
+
+---
+
+## Introducing a universal NumPy interface for various NumPy-compatible libraries
+
+A general NumPy wrapper can be developed in PySCF to provide a universal interface
+to access NumPy functions. This interface would enable automatic switching to
+`jax.numpy` or `torch` functions depending on the runtime context.
+
+This interface can be imported as::
+
+  import pyscf.numpy as fnp
+
+
+---
+
+## Allowing extensions written in C++

source/contributor/index.md

@@ -1,6 +1,6 @@
 # Contributor Guide 
 
-A more complete Contributor Guide for PySCF is planned for the future.
+A more complete Contributor Guide and planned tasks for PySCF for the future.
 
 ```{toctree}
 :caption: Style guide
@@ -9,3 +9,13 @@ A more complete Contributor Guide for PySCF is planned for the future.
 code-standard
 ```
 
+```{toctree}
+:caption: API change proposals
+:maxdepth: 1
+
+api-v3.rst
+```
+
+```{toctree}
+:caption: Feature requests
+```