Update documentation with coding guidelines
This commit:
* adds a reference to the Spring Code Style wiki page in the main
CONTRIBUTING document
* updates the link to the Spring team in README
* adds a note regarding Intellij IDEA 13 issues
Support String contentType headers in spring-messaging

After this change DefaultContentTypeResolver supports String-based
"contentType" header values in addition to MimeType-based.

Issue: SPR-11461