Merge pull request iluwatar#119 from iluwatar/iluwatar-patch-1

Update contribution instructions

CONTRIBUTE.md

@@ -13,3 +13,36 @@ For new snippets the general implementation steps are:
 * Submit pull request against `master` branch.
 
 In case you are a first-time contributor and would like to become familiar with working in Github, check out the tutorial in this [repository](https://github.com/firstcontributions/first-contributions).
+
+## Building the project
+
+Building the project's `master` branch is straightforward.
+
+```
+./gradlew clean
+./gradlew build
+```
+
+
+## Checkstyle
+
+The project uses Checkstyle and sometimes the build can fail due to rule violations, e.g.
+
+```
+> Task :checkstyleTest FAILED
+[ant:checkstyle] [WARN] /home/ilkka/30-seconds-of-java/src/test/java/math/GenerateRandomNumbersSnippetTest.java:24: 'package' should be separated from previous statement. [EmptyLineSeparator]
+
+FAILURE: Build failed with an exception.
+
+* What went wrong:
+Execution failed for task ':checkstyleTest'.
+> Checkstyle rule violations were found. See the report at: file:///home/ilkka/30-seconds-of-java/build/reports/checkstyle/test.html
+  Checkstyle files with violations: 1
+  Checkstyle violations by severity: [warning:1]
+```
+
+The violations are easy check and fix using the indicated report file. IDEs also have Checkstyle plugins for better developer experience.
+
+## License check
+
+The project is configured to check that the source code files have proper license headers. When new source code files are added, they should also contain the license headers. It's easy to copy-paste them from other files or even better to use the Gradle license formatting plugin e.g. `./gradlew licenseFormat` to add missing license headers.