Add stable-baselines-tf2 to project list page.

[sophiagu]

Description

closes #984

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have ensured pytest and pytype both pass (by running make pytest and make type).

[Miffyli]

Please fill in the checklist (e.g. update changelog and include username).

[sophiagu]

The latest release date in changelog is 2020-08-05. Should I add changes under this release date or create a new release date?

[Miffyli]

@araffin Could you answer the above?

[sophiagu]

There is a unit test failure which seems to be irrelevant to my changes: https://travis-ci.com/github/hill-a/stable-baselines/jobs/377081203

[araffin]

The latest release date in changelog is 2020-08-05. Should I add changes under this release date or create a new release date?

I added a new section for it ;)

[sophiagu]

pending review?

[Miffyli]

Could you add your alias to the changelog as instructed in the contributor's guide. After that this seems ready for merging :)

Edit: I added them quick and will merge after checks pass. Cheers for the PR!

[sophiagu]

Thx!!