Build instructions do not appear to work in src package

[Former user]

Steps to reproduce:

1. Download the src package from http://jastadd.org/releases/jastadd2/2.3.4/
2. Run gradle wrapper (Is the wrapper script not normally distributed to ensure the right version of gradle is installed?)
3. Run ./gradlew or ./gradlew release as per the README.
4. Nothing happens; the latter gives an error that release is not a Task.

[Jesper Öqvist]

The gradle wrapper is included in the jastadd2 repository, so ./gradlew should work in a fresh checkout.

I do get an error when I run ./gradlew release, but the error is not that the task is missing:

* What went wrong:
Execution failed for task ':release'.
> You should add -PnewVersion=<VERSION> so the release task embeds the correct version string!

We should update the README to include -PnewVersion=XX for the release example.

[Jesper Öqvist]

Update README

see #327

→ <<cset 8abc63ad998c>>