Fill up the "Building and executing" Wiki page

[SergioLuis]

Developer: @mikelcaz Reviewer: @SergioLuis @jhvargas3112 @ugedo

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Fill the Building and executing Wiki page. The work is not started.

Building should be really easy using Gradle, but bear in mind in the instructions the following:

• It is necessary to have the 1.8 JDK installed.
  • The JAVA_HOME environment variable should point to the JDK path.
• It is necessary to have the Android SDK installed, with the 27.? build tools.
  • The ANDROID_HOME environment variable should point to the SDK path.

The instructions should also cover how to install applications from outside the Play Store, and a link to the Releases section of the repository (we'll upload the APKs there).

We'll probably have DEBUG and RELEASE builds, so maintaining that Wiki page will be a work in progress until the end of the project, so feel free to delay this task as much as possible.

[mikelcaz]

Installing instructions will be completed in the memory, for now. Have a little sense here.

The part of the enviroment variables are polemic right now, as CHIPEIT seems to build powered by luck in my system. I have to find why: it shouldn't do that.