Expand Issue Tracking docs to include triage, Chrome instructions

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2255133003
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2255133003

Review-Url: https://codereview.chromium.org/2255133003
1 file changed