Add metadata-convention-compliant metadata file (SCP-2832)

[jlchang]

To improve metadata convention adoption, provide an example metadata-convention-compliant metadata file. This example file does not contain biosample_type (in anticipation of making biosample_type an optional metadata, SCP-2806, to be completed this sprint), but has all other required metadata plus cell type and an example (non-convention) numeric annotation.

This satisfies SCP-2832.

[codecov[bot]]

Codecov Report

Merging #111 into master will not change coverage. The diff coverage is n/a.

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[jlchang]

Content looks good, but please add "metadata" somewhere in the file name, e.g. perhaps mc_compliant_metadata_example.txt. That will help users (and us) recall at a glance what this file is for.

Updated file name in c728f23. Thanks for the suggestion!