message: Add docs for IS_LOCAL flag
diff: @@ -222,6 +222,10 @@ Here's the priority of the files for the `dev` stage. Files on the left have mo
  SST will never modify any environment variables that have already been set. In particular, if there is a variable in your .env file which collides with one that already exists in your environment, then that variable will be skipped.
  +#### Built-in environment variables
  +
  +`IS_LOCAL` is set to `true` by default when running inside `sst start`.
  +
  #### Should I commit my .env files?
  The `.env` and `.env.$STAGE` files should be included in your repository, but `.env.local` and `.env.$STAGE.local` shouldn't, as `.env*.local` are intended to be ignored through `.gitignore`.
  