Add a prerequisites section

[ehogan]

Some of the steps in the ESMValCore release documentation could not be completed until completing other tasks or asking specific people to complete tasks that could have been completed prior to starting the release.

These tasks should be added to a prerequisites sections (perhaps an admonition?) in the ESMValCore release documentation.

Prerequisites

Before performing an ESMValCore release, the release manager must have:

1. The feature freeze date for ESMValCore.
   • The release manager is required to perform tasks two weeks before the feature freeze date.
2. An ESMValCore Milestone for the release.
   • A milestone should already exist and be populated with Issues and PRs intended for inclusion in the release.
3. A previous release manager.
   • The previous release manager will act as a "buddy" and help guide the release manager through the release process.
4. Membership of the ESMValTool-DevelopmentTeam on GitHub.
5. Accounts for the following services:
   • Read the Docs
   • Docker Hub
   • PyPI (but only if the automatic upload fails)
   • DKRZ/Levante (for recipe testing)
6. Membership of the following (ask the previous release manager to provide membership):
   • Read the Docs: esmvalcore
   • Docker Hub: esmvalgroup
   • PyPI: esmvalgroup (but only if the automatic upload fails)
   • GitHub: conda-forge/esmvalcore-feedstock
   • The ESMValTool VM at DKRZ
7. A local clone of the ESMValCore repository.
8. A Conda environment that contains pygithub (for creating the draft release notes).
9. A Personal Access Token (PAT) (for creating the draft release notes).