Provide orientation re: Seurat & Scanpy

[lgeistlinger]

• this will be helpful for users / participants already prominently on the main page of the course

• this is a requirement for being included in The Carpentries Incubator (Process for moving the module into the carpentries incubator #12):
  Your lesson/proposed topic should be distinct from existing official and community developed lessons (in domain of expertise, tools and skills taught/used, target audience, etc). (there is a community-developed scRNA-seq lesson that uses a bit of scanpy and a seurat-based proposal)

[lgeistlinger]

took a shot at this in eb69cc0