Provide build instructions

[aalmiray]

Cloning and building the project leads to errors due to missing Project properties, ideally set as secrets in ~/.gradle/gradle.properties.

Please provide a BUILD.md stating build requirements such as:

• JDK version
• secrets

Moreover, there could be a check performed by the build collecting all missing properties and print them out with a helpful message. I ended pup running the build 5 times before it succeeded.

Missing properties are:

• ossrhUsername
• ossrhPassword
• ghUser
• ghToken