Add doc about adding gradle-cache to build pipeline

[renedupont]

One can improve the build pipeline time by adding a Gradle cache. This way, gradlew can use
previously downloaded distributions instead of downloading them from the internet on every build.

A how to guide about this could be added to the readme.

I provided a PR for this: #1044

[renedupont]

We decided that this fits better in an internal documentation platform.