Added conda-forge details to installation instructions

[michaelcdevin]

Description

The README instructions assume the user has already added conda-forge as a channel in the conda environment, which may not be clear to new users of Python and/or conda. This adds a sentence with a link to the conda-forge website specifying how to add the channel. Closes #261.

Type of PR

• Bug fix
• New feature
• Documentation
• Other: (specify)

Checklist for PR

• Authors read the contribution guidelines
• The pull request is from an issue branch (not main) on your fork, to the main branch in WecOptTool.
• The authors have given the admins edit access
• All changes adhere to the style guide including PEP8, Docstrings, and Type Hints.
• Modified the documentation if applicable
• Modified or added a new test
• All tests pass
• Reference or close any relevant issues