README.md: Document OrderLine.increment and decrement
README.md: Document OrderLine.increment and decrement

The in-flight portion of the implementation here makes
increment/decrement more responsive by allowing folks to fire them
multiple times without waiting for the round-trip response from the
API between calls.  For example, if quantity-ordered is one:

  increment()     quantity-ordered set to 2
                  request sent with quantity-ordered of 2
                  $quantity-ordered set to 2
  increment()     quantity-ordered and $quantity-ordered set to 3
  increment()     quantity-ordered and $quantity-ordered set to 4
    ... time passes...
                  response with quantity-ordered of 2
    saveUntilItSticks compares quantity-ordered with $quantity-ordered, 2 != 4
                  request sent with quantity-ordered of 4
    ... time passes...
                  response with quantity-ordered of 4
    saveUntilItSticks compares quantity-ordered with $quantity-ordered, 4 == 4
                  remove $quantity-ordered value, now that we're not in-flight

azurestandard/azure-angular-providers,azurestandard/azure-angular-providers