README: add versions for 2.8.0

Author: omelkonian

README.md

@@ -13,5 +13,6 @@ We mirror the version numbers of [agda-stdlib](https://github.com/agda/agda-stdl
 | [v2.6.4](https://github.com/agda/agda/releases/tag/v2.6.4) | [v2.0](https://github.com/agda/agda-stdlib/releases/tag/v2.0) | [v2.0](https://github.com/agda/agda-stdlib-classes/releases/tag/v2.0) |
 | [v2.7.0](https://github.com/agda/agda/releases/tag/v2.7.0) | [v2.1.1](https://github.com/agda/agda-stdlib/releases/tag/v2.1.1) | [v2.1.1](https://github.com/agda/agda-stdlib-classes/releases/tag/v2.1.1) |
 | [v2.7.0.1](https://github.com/agda/agda/releases/tag/v2.7.0.1) | [v2.2](https://github.com/agda/agda-stdlib/releases/tag/v2.2) | [v2.2](https://github.com/agda/agda-stdlib-classes/releases/tag/v2.2) |
+| [v2.8.0](https://github.com/agda/agda/releases/tag/v2.8.0) | [v2.3](https://github.com/agda/agda-stdlib/releases/tag/v2.3) | [v2.3](https://github.com/agda/agda-stdlib-classes/releases/tag/v2.3) |
 
 Minor revisions will append to these major versions (e.g. `v2.1.1b` or `v2.1.1.10`).