Suggest installing CLIMADA via conda in installation instructions

[peanutfun]

Changes proposed in this PR:

• Instead of building the climada_env and installing CLIMADA via pip, the "Simple Instructions" in the installation instructions now advise users to simply install CLIMADA via conda. This only requires a single command.
• Advertise installation with conda also in the README.md.

This PR fixes #673

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• [ ] Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[chahank]

Thanks for the conda install @peanutfun !

[bguillod]

Out of curiosity: Why do we now want to recommend to users to install CLIMADA via conda, which is (in my experience at least) much more cumbersome than via pip?

[chahank]

CLIMADA can only really be installed using conda at the moment (the climada_env needs to be made with conda). So I am not sure to understand what you mean?

[peanutfun]

@chahank Did you finish your review?

[chahank]

Yes, thanks!