Improve README file

The file includes two improvements:
- Rather than having a direct mention to the copyright holders in the
README file, a reference to the COPYRIGHT and COPYRIGHT.postgresql files
is added to provide some redirection.  This simplifies a little bit the
COPYRIGHT date refreshes.
- The list of items is organized my numbers, which is less confusing
when reading them in an editor.

Backpatch-through: 14

Author: michaelpq

README.md

@@ -10,19 +10,18 @@ The planner does its best to select the best execution plan, but it is far
 from perfect, since it may not count some data properties, like correlation
 between columns.
 
+Copyright and license information can be found in the files COPYRIGHT and
+COPYRIGHT.postgresql.
+
 For more details, please see the various documentations available in the
 **docs/** directory:
 
 1. [Description](docs/description.md)
-1. [The hint table](docs/hint_table.md)
-1. [Installation](docs/installation.md)
-1. [Uninstallation](docs/uninstallation.md)
-1. [Details in hinting](docs/hint_details.md)
-1. [Errors](docs/errors.md)
-1. [Functional limitations](docs/functional_limitations.md)
-1. [Requirements](docs/requirements.md)
-1. [Hints list](docs/hint_list.md)
-
-* * * * *
-
-Copyright (c) 2012-2022, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
+2. [The hint table](docs/hint_table.md)
+3. [Installation](docs/installation.md)
+4. [Uninstallation](docs/uninstallation.md)
+5. [Details in hinting](docs/hint_details.md)
+6. [Errors](docs/errors.md)
+7. [Functional limitations](docs/functional_limitations.md)
+8. [Requirements](docs/requirements.md)
+9. [Hints list](docs/hint_list.md)