Feature/proper rst (#464)

* Use rst title syntax for Unreleased section of changelog, like the other sections have, rather than a horizontal line

* Convert CONTRIBUTING.rst from md -> rst, as the filename suggests it to be

CHANGELOG.rst

@@ -1,5 +1,5 @@
 Unreleased
------
+----------
 * Exceptions: Changed ProcessExecutionError's formatting to be more user-friendly (`#456 <https://github.com/tomerfiliba/plumbum/pull/456>`_)
 * Commands: support for per-line timeout with iter_lines (`#454 <https://github.com/tomerfiliba/plumbum/pull/454>`_)
 * Commands: support for piping stdout/stderr to a logger (`#454 <https://github.com/tomerfiliba/plumbum/pull/454>`_)

CONTRIBUTING.rst

@@ -1,14 +1,16 @@
-# Contributing to Plumbum
+Contributing to Plumbum
+=======================
 
+General comments
+----------------
 
-## General comments
+Pull requests welcome! Please make sure you add tests (in an easy ``pytest`` format) to the tests folder for your fix or features. Make sure you add documentation covering a new feature.
 
-Pull requests welcome! Please make sure you add tests (in an easy `pytest` format) to the tests folder for your fix or features. Make sure you add documentation covering a new feature.
-
-## Adding a language
+Adding a language
+-----------------
 
 Plumbum.cli prints various messages for the user. These can be localized into your local language; pull requests adding languages are welcome.
 
-To add a language, run `./translations.py` from the main github directory, and then copy the file `plumbum/cli/i18n/messages.pot` to `plumbum/cli/i18n/<lang>.po`, and add your language. Run `./translations.py` again to update the file you made (save first) and also create the needed files binary file.
+To add a language, run ``./translations.py`` from the main github directory, and then copy the file ``plumbum/cli/i18n/messages.pot`` to ``plumbum/cli/i18n/<lang>.po``, and add your language. Run ``./translations.py`` again to update the file you made (save first) and also create the needed files binary file.
 
 See `gettext: PMOTW3 <https://pymotw.com/3/gettext/>`_ for more info.