Suggest how to solve PlatformValidationError

[maresb]

Closes #281

Example error message:

conda_lock.errors.PlatformValidationError: The lockfile conda-lock.yml does not contain
a solution for the current platform linux-64. The lockfile only contains solutions for
the following platforms: osx-64. In order to add support for linux-64, you must
regenerate the lockfile. Either add the following section to your environment.yml:

platforms:
- linux-64
- osx-64

or add the following arguments to your conda-lock command:

--platform=linux-64 --platform=osx-64

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