Add references and more explanation to contaminants section

[JoBBurt]

Flushed out the contaminants database section with more background regarding the effect of contaminants on false protein identification and quantitation. Also added in a few more contaminant databases for cell culture experiments with fetal bovine serum.

Please provide a description of your changes:

Please check the following about your suggested changed:

• [ ] The new text has one sentence per line
• [ ] preprints that are cited are listed as an issue for discussion and peer review

[jessegmeyerlab]

This is awesome thank you! I will merge this. When you finish your other pull request please add to that one an update to the metadata.yml with your author info.