doc: add screencast to docs

[rakyll]

This is a tracking bug to add screencasts to the docs.

A brief draft of the content we want to cover with the screencasts, these are not sorted by priority at this moment - please contribute to the discussion of prioritization:

• Installation
  • Platform dependent
• Environment set-up
  • Platform dependent
  • GOPATH / src/pkg/bin etc
  • go version
  • go help
• Writing "Hello, world!"
  • Where does it go?
  • Editors, plugins, etc (link to them)
  • Importing packages in the std lib / How to access documentation? golang.org/pkg/
  • go install/run/build
• Writing your first library
  • Package names and how to import them
  • Documentation
• Using non stdlib packages
  • Importing non stdlib packages
  • go get (-u)
• Testing
  • go test
  • Code coverage
• Managing your GOPATH
  • go list
  • Folder structure
  • Multiple GOPATHs
• Managing dependencies
  • Vendoring?
  • Versioning? Semver
• Contributing to GitHub repos
  • Forks and PRs
• Contributing to the Go project
  • Set up
  • Gerrit
  • Contributing guidelines
• Debugging
• Performance profiling
  • Benchmarks
  • Load testing
  • Pprof
  • Flame Graphs
• Execution tracer

Constraints on videos:

• Platform dependent steps should be contained into different video variations
• Videos should be short (target ~2 minutes, absolute max 5 minutes)
• Each video should mirror a specific document, to improve cross linking

/cc @campoy @goinggo @jessfraz

[ScottMansfield]

How will these be maintained over time?

[rakyll]

Good question. We are keeping things small and brief to make it possible reshoot them. Also, videos are going to be supplementary only. If we cannot update an outdated video, it should be safe to just remove it and depend on the text.