add conda-forge as channel to env create command

[radRoy]

In my case, I did not want to install Miniforge3. I did not understand why it's necessary as I already have conda and mamba installed.

When trying your mamba create command, pyimagej could not be found. A simple -c conda-forge added to the create command solves the problem without the need for Miniforge3, which requires admin priveleges to install.

[ctrueden]

@radRoy Installing Miniforge3 is not strictly necessary, as you point out. However, we recommend it because it is the easiest path forward for new users. We deeply appreciate it when people contribute patches to the project. In this case, however, we very much want to avoid making the installation instructions any more complicated than they need to be. For users such as yourself who know what they are doing with Conda already, we reasoned that you would already know about conda-forge and have the channel activated. That said, to ensure that everyone has the needed conda-forge channel activated and prioritized, I added instructions on doing so with e807b9028e63df7043071430a093fd8c796f9987. I tried to make it as succinct as possible, and also indicate that if you install Miniforge3 then you don't need to do it.