Add .gitignore for Gradle Plugin module

[vpavic]

If one attempts to compile Spring Boot project in IntelliJ, there are errors in spring-boot-gradle-plugin module unless that module is imported as Gradle project. Doing so (by default) generates out/ directory with compilation output and these files are not ignored by Git ATM.

This PR updates .gitignore to add out/ pattern adds .gitignore for Gradle Plugin module.

[snicoll]

Thanks for the PR.

I am not sure about that pattern and I'd like @wilkinsona to have a look to it first.

[snicoll]

I am not super keen to add such a global pattern. Perhaps we should make that a bit more specific? See also spring-io/initializr#657

[vpavic]

That one's a quite common .gitignore entry for IntelliJ. For instance, see GitHub's own Jetbrains .gitignore template. That being said, I'm fine with it being updated to something more specific if you prefer.

[wilkinsona]

I agree on making this more specific. In fact, I'd do the same for bin too. It's Eclipse's equivalent of out and also only applies to the Gradle plugin. While we're at it, we could do something for build too.

There's more than one way to make the ignore more specific, but I think I'd create a separate .gitignore file in the Gradle plugin's directory. That'd mean the new file had the following contents:

/bin/
/build/
/out/

[vpavic]

Thanks for the feedback - I've updated the PR according to separate .gitignore file suggestion.