Stress that .lock extension is necessary

[klieret]

It took me a while to realize that conda/mamba refuses to deal with conda-lock files if they have a yaml ending (throwing a very ugly decoding error).

This PR tries to make this more clear in the readme (though it is more of an issue with conda/mamba, I guess).

(Also see mamba-org/provision-with-micromamba#106)

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[maresb]

Thanks a lot for this! I think such small documentation changes make a big difference.

[mariusvniekerk]

We can also potentially just attempt to parse every file initially with the conda lock format and if that works treat it as such. That minimizes surpise factors

[klieret]

My major dissatisfaction was that the error message was very unhelpful. So just catching & reraising the error with a bit of added bit of information ("Please note that file extensions matter: For example, if you try to read a .... file... use the conda-lock.yml ending") etc. would already add a lot, without changing any logic.

[maresb]

@klieret, thanks again for the contribution. We're very happy to accept PRs which improve the error handling. For example, I just added #282 this morning.

[klieret]

Thanks @maresb! I plan follow up with a PR (though I'm a bit short on time atm)