Add commit message requirements to CONTRIBUTING.md
Add commit message requirements to CONTRIBUTING.md

1. Separate subject from body with a blank line
2. Limit the subject line to 50 characters
3. Capitalize the subject line
3. Do not end the subject line with a period
4. Use the imperative mood in the subject line
5. Wrap the body at 72 characters
6. Use the body to explain what and why vs. how

evpobr/libsndfile,evpobr/libsndfile,erikd/libsndfile,libsndfile/libsndfile,evpobr/libsndfile,libsndfile/libsndfile,evpobr/libsndfile,evpobr/libsndfile,erikd/libsndfile,libsndfile/libsndfile,erikd/libsndfile,erikd/libsndfile,libsndfile/libsndfile,libsndfile/libsndfile,erikd/libsndfile