Condense the Google Java Style Guide into `conventions.md` and finish off Gradle documentation

[Diptonil]

• The conventions.md file in the directory of further reading is lacking as of now. Condense the whole manifest of the Google Java Style Guide into the file in the format that has already been laid down.
• Documenting the gradle.md file needs to be finished off so as to impart a thorough and solid understanding of the build system. Refer to the maven.md under the Further Reading section for better understanding on how to tackle the documentation.
• Elaborations and alterations in reasonable limits are acceptable.
• Contributors are expected to be thorough in the English language.