Fix hyperlinks in README (#471)

Author: BenRKarl

README.rst

@@ -30,19 +30,15 @@ configure, and use this client library.
 
 For Python 3.6 Users
 --------------------
-Version [12.0.0](https://pypi.org/project/google-ads/12.0.0/) of this library
-is the last version that is compatible with Python 3.6. It contains support for
-[`v6`](https://developers.google.com/google-ads/api/reference/rpc/v6/overview),
-[`v7`](https://developers.google.com/google-ads/api/reference/rpc/v7/overview),
-and [`v8`](https://developers.google.com/google-ads/api/reference/rpc/v8/overview)
-of the Google Ads API. The newest API version, `v8`, will be supported until
-the Spring of 2022. Given that the [EOL](https://endoflife.date/python) for
-Python 3.6 is December 23, 2021, we encourage our users to upgrade to Python 3.7
-or above as soon as possible to avoid issues. Users who cannot upgrade can
-continue to safely use version 12.0.0 until `v8` of the Google Ads API is
-deprecated. Please follow the
-[Google Ads Developer Blog](https://ads-developers.googleblog.com/) for
-announcements of the specific deprecation dates for the above API versions.
+Version `12.0.0`_ of this library is the last version that is compatible with
+Python 3.6. It contains support for `v6`_, `v7`_, and `v8`_ of the Google Ads
+API. The newest API version, `v8`_, will be supported until the Spring of 2022.
+Given that the `EOL`_ for Python 3.6 is December 23, 2021, we encourage our
+users to upgrade to Python 3.7 or above as soon as possible to avoid issues.
+Users who cannot upgrade can continue to safely use version `12.0.0`_ until
+`v8`_ of the Google Ads API is deprecated. Please follow the
+`Google Ads Developer Blog`_ for announcements of the specific deprecation
+dates for the above API versions.
 
 Miscellaneous
 -------------
@@ -70,3 +66,9 @@ Authors
 .. _Ben Karl: https://github.com/BenRKarl
 .. _Andrew Burke: https://github.com/AndrewMBurke
 .. _Laura Chevalier: https://github.com/laurachevalier4
+.. _12.0.0: https://pypi.org/project/google-ads/12.0.0/
+.. _v6: https://developers.google.com/google-ads/api/reference/rpc/v6/overview
+.. _v7: https://developers.google.com/google-ads/api/reference/rpc/v7/overview
+.. _v8: https://developers.google.com/google-ads/api/reference/rpc/v8/overview
+.. _EOL: https://endoflife.date/python
+.. _Google Ads Developer Blog: https://ads-developers.googleblog.com/