Automatically add mybinder badge on PRs

See https://github.com/jupyterhub/binder/issues/212#issuecomment-687706341

Update README.md

Two small — though important — changes

  1. Since [the release of conda 4.6](https://www.anaconda.com/blog/conda-4-6-release), the [recommended way to activate a conda env](https://docs.conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#activating-an-environment) is to use `conda activate <myenv>`.
  2. The [conda docs say](https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#exporting-an-environment-file-across-platforms) that the correct way to export an env is to use the `--from-history` flag.  This only includes packages that you’ve explicitly asked for, and only "pins" versions when you asked to do so.  If you use this flag, the "macOS-specific" packages probably won't appear.

OS-specific packages correction to README

Update README.md

We don't make anaconda available

Fix example