docs: Update CONTRIBUTING.md with commit message guidelines

[congminh1254]

Clarify commit message guidelines and examples for scopes.

[coveralls]

Pull Request Test Coverage Report for Build #5207

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 77 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 13.665%

---

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/BoxAPIConnection.java	77	79.05%

Totals
Change from base Build #5142:	0.0%
Covered Lines:	8374
Relevant Lines:	61282

---

💛 - Coveralls