Sample ID for design file

User had error using example design file as template.

Example has SI-P03-C9 as the ID but current versions of the sample barcodes have the prefix SI-GA-

• Replace ID in example with a SI-GA- code
• In readme.md and astrocyte.yaml please add a link for the current ID's and their barcodes https://s3-us-west-2.amazonaws.com/10x.files/supp/cell-exp/chromium-shared-sample-indexes-plate.csv but you don't need to bring huge attention to it
• In readme.md and astrocyte.yaml please comment on the fact that the last 2 characters is the well position of the 96-well plate that the sample barcode kit is sold in (A1 to H12)