Include comments from PR discussion, minor style change

CONTRIBUTING.rst

@@ -5,19 +5,28 @@ The SciPy lecture notes are a community-based effort and require constant
 maintenance and improvements. New contributions such as wording
 improvements or inclusion of new topics are welcome. 
 
-Contributing guide
-------------------
+To propose bugfixes or straightforward improvements to the notes, see the
+contribution guide below.
+
+For new topics, read the objectives first and `open an issue on the GitHub
+project <https://github.com/scipy-lectures/scipy-lecture-notes/issues>`_ to
+discuss it with the editors.
+
+
+Objectives and design choices for the lecture notes
+---------------------------------------------------
 
 Contributors should keep the following objectives and design choices of
 the SciPy lecture notes in mind.
 
 Objectives:
 
-* The SciPy lecture notes provide a self-contained introduction to Python
-  and its primary computational packages, the ”SciPy stack“.
-* They provide an introduction to a selection of computational libraries.
-  Currently, pandas, statmodels, seaborn, scikit-image, scikit-learn,
-  sympy, and mayavi are discussed.
+* Provide a self-contained introduction to Python and its primary computational
+  packages, the ”SciPy stack“.
+* Provide tutorials for a selection of widely-used and stable computational
+  libraries.
+  Currently, we cover pandas, statmodels, seaborn, scikit-image,
+  scikit-learn, sympy, and mayavi.
 * Automated testing is applied to the code examples as much as possible.
 
 Design choices:
@@ -27,6 +36,10 @@ Design choices:
 * An idomatic style should be followed, e.g. ``import numpy as np``,
   preference for array operations, PEP8 coding conventions.
 
+
+Contributing guide
+------------------
+
 The directory ``guide`` contains instructions on how to contribute:
 
 .. topic::  **Example chapter**