Link /addons from new subsection on /contributing page (#2967)

* link /addons page in new subsection on /contributing page

* rename mpmaster branch to upstream

docs_rst/contributing.rst

@@ -41,9 +41,9 @@ Direct contributions to pymatgen main distribution
       pip install -e '.[dev,optional]'
 
 6. Code (see `Coding Guidelines`_). Commit early and commit often. Keep your code up to date. You need to add the main
-   repository to the list of your remotes. Let's name the upstream repo as mpmaster (materialsproject master)::
+   repository to the list of your remotes. Let's name the upstream repo as upstream (materialsproject master)::
 
-       git remote add mpmaster git://github.com/materialsproject/pymatgen.git
+       git remote add upstream git://github.com/materialsproject/pymatgen.git
 
    Make sure your repository is clean (no uncommitted changes) and is currently on the master branch. If not, commit or
    stash any changes and switch to the master::
@@ -52,7 +52,7 @@ Direct contributions to pymatgen main distribution
 
    Then you can pull all the new commits from the main line::
 
-      git pull mpmaster master
+      git pull upstream master
 
    Remember, pull is a combination of the commands fetch and merge, so there may be merge conflicts that may need to be
    manually resolved.
@@ -94,13 +94,11 @@ under the pymatgen.analysis, pymatgen.ext and and pymatgen.io namespaces and not
 pymatgen root namespace is meant for development of broad classes of functionality. If in doubt, please consult with
 the pymatgen maintainers. The benefits of writing an add-on for pymatgen are:
 
-* You control the development and distribution of the add-on. You also get full recognition of your work.
+* You control the development and distribution of the add-on. You also get full recognition for your work.
 * The add-on does not affect the main pymatgen distribution and end users have a choice of whether to install the
   add-on or not via `pip install pymatgen-analysis-addon`.
 * Once installed, the add-on functions exactly like a part of pymatgen in that the imports are still via
   `from pymatgen.analysis.addon import *`.
-* We will help your add-on gain recognition via our `listing of pymatgen add-ons </addons>`_. We have plans to develop
-  this into a full-fledged searchable database of add-ons.
 
 The namespaces provide an important clue what kind of contributions are suitable for add-ons.
 
@@ -121,6 +119,11 @@ The add-on architecture therefore provides flexibility for broad expansion of sc
 community by loosening up the tight control in the main repository, which is bottlenecked by the small team maintaining
 it.
 
+Existing add-ons and ecosystem tools
+------------------------------------
+
+We help add-ons gain visibility and recognition on our `add-ons page </addons>`_. Feel free to suggest additions to this list via `GitHub PR <https://github.com/materialsproject/pymatgen/edit/master/docs_rst/addons.rst>`_.
+
 Coding Guidelines
 -----------------