Fix incomplete sentence in README.md for Gradle plugin

[SableRaf]

Expanded explanation of Gradle plugin's role (sentence was incomplete).

Before:

You do not need to learn Gradle to use Processing in the P

After:

You do not need to learn Gradle to use Processing in the PDE, but if you feel ready to take your sketches beyond the PDE and into development environments such as IntelliJ or Eclipse, this plugin helps bridge the gap between Processing and standard Java workflows.