application-travis.properties: add a link to Travis CI documentation.

No functional changes.

[ci skip]

Author: php-coder

src/main/resources/application-travis.properties

@@ -1,5 +1,6 @@
 spring.profiles: travis
 
+# Details about MySQL configuration on Travis CI: https://docs.travis-ci.com/user/database-setup/#MySQL
 spring.datasource.url: jdbc:mysql://localhost:3306/mystamps?logger=com.mysql.jdbc.log.Slf4JLogger&useSSL=false&logSlowQueries=true&slowQueryThresholdMillis=500&autoSlowLog=false&explainSlowQueries=true&characterEncoding=UTF-8
 spring.datasource.username: travis
 spring.datasource.password: