Add list of websites that use creativecommons-base as their base theme

## Problem

We don't currently have a list of CC websites that use creativecommons-base as a base theme.

## Description

Add a list of all CC websites that use creativecommons-base as a base theme to the project README

## WIP table

The following table shows Creative Commons WordPress projects and indicates whether the theme inherits from `creativecommons-base`. The table will be used as the basis for the final list of projects inheriting from `creativecommons-base` and includes negative cases for thoroughness.

The following themes inherit from `creativecommons-base`:
| Project | is child-them | not child-theme |
| ------------- |:-------------:|:-----:|
| [certificate][certificate] | :heavy_check_mark: | |
| [commoners][commoners] | :heavy_check_mark: | |
| [new-creativecommons.org][new-creativecommons.org] | | :heavy_check_mark: |
| [open4us.org][open4us.org] | | :heavy_check_mark: |
| [openpolicynetwork.org][openpolicynetwork.org] | | :heavy_check_mark: |
| [sotc-2016-wordpress][sotc-2016-wordpress] | | :heavy_check_mark: |
| [wp-theme-cc-chapter][wp-theme-cc-chapter] | | :heavy_check_mark: |
| [wp-theme-cc-commoners][wp-theme-cc-commoners] | | :heavy_check_mark: |
| [wp-theme-cctoolkit][wp-theme-cctoolkit] | | :heavy_check_mark: |
| [wp-theme-openglam][wp-theme-openglam] | | :heavy_check_mark: |
| [wp-theme-summit][wp-theme-summit] | :heavy_check_mark: | |
| [wp-theme-creativecommons.org][wp-theme-creativecommons.org] | :heavy_check_mark: | |

[commoners]: https://github.com/creativecommons/commoners
[certificate]: https://github.com/creativecommons/creativecommons-certificate/blob/b2967296b0324f33d1be4c041954513363191bcc/composer.json#L21
[new-creativecommons.org]: https://github.com/creativecommons/new-creativecommons.org
[open4us.org]: https://github.com/creativecommons/open4us.org
[openpolicynetwork.org]: https://github.com/creativecommons/openpolicynetwork.org
[sotc-2016-wordpress]: https://github.com/creativecommons/sotc-2016-wordpress
[wp-theme-cc-chapter]: https://github.com/creativecommons/wp-theme-cc-chapter
[wp-theme-cc-commoners]: https://github.com/creativecommons/wp-theme-cc-commoners
[wp-theme-cctoolkit]: https://github.com/creativecommons/wp-theme-cctoolkit
[wp-theme-openglam]: https://github.com/creativecommons/wp-theme-openglam
[wp-theme-summit]: https://github.com/creativecommons/wp-theme-summit
[wp-theme-creativecommons.org]: https://github.com/creativecommons/wp-theme-creativecommons.org/blob/375cafbaccf03eded02f8f7f422e02e97833515a/composer.json#L20

## Additional context
This is related to the possibility of deprecating Queulat (#69), on which this base theme depends.