Note: do not ignore Gradle wrapper files

[v-jonathangu]

App Center Build Service does not find the project when you ignore the Gradle wrapper files, add a quick notice to not ignore those files on the docs, example:
ignore these files:

will show this on App Center Build:

if those files are ignored App Center Build might not find them, as some IDE generates them, like Android Studio the users may ignore them, for example Android Flutter projects (which aren't yet supported officially, see: microsoft/appcenter#67 ) ignore it by default, this PR adds a quick note that warns the user to not ignore them.

Quick Notes:

• this is also discussed on the Flutter GitHub as many users ignore them.
• this issue is not specific to flutter many users may ignore it without knowledge of this.
• it should be a note about this also on the sample flutter script Note: do not ignore Gradle wrapper files microsoft/appcenter#2375

[PRMerger20]

@v-jonathangu : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[stephan-markovic]

I would like to propose some changes to the text:
The App Center tracks the project by searching the gradle (and gradlew) directory files of the Android project. Please do not include these files in the project .gitignore as App Center Build will not be able to find them.

[PRMerger14]

@v-jonathangu : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[lucen-ms]

#sign-off