Tweak readme

Few things:

  - SBT has new, nice site
  - we switched to SBT 0.13.5+ (it is required, 0.13.4 or earlier won't work due to use of `JvmPlugin`)
  - directory to place code in changed in last commit
  - directory to place assets in changed veeery long time ago
  - calling update is kind of common knowledge for SBT, also updated does not seem to trigger library unpacking, which is triggered by compile or manually - so I think those few lines aren't necessary