[Minor] Quick edits to .md files

ghstack-source-id: d0fbd3d
Pull-Request-resolved: #2931

Author: Vincent Moens

CONTRIBUTING.md

@@ -17,6 +17,7 @@ or the git version of the library:
 ```
 pip install git+https://github.com/pytorch/tensordict
 ```
+This command requires `pybind11[global]` and cmake to be installed.
 
 Once cloned, make sure you install torchrl in develop mode by running
 ```

README.md

@@ -895,7 +895,7 @@ git clone https://github.com/pytorch/rl
 ```
 and don't forget to check out the branch or tag you want to use for the build:
 ```bash
-git checkout v0.4.0
+git checkout v0.8.0
 ```
 
 Go to the directory where you have cloned the torchrl repo and install it (after