Update document to explain how to use with Travis CI

[KengoTODA]

In my understanding, we can use following explanation:

Run incremental analysis in Travis CI

Create a profile activated only in the build on Travis CI. For instance:

<profile>
  <id>travis</id>
  <activation>
    <property>
      <name>env.TRAVIS</name>
    </property>
    <property>
      <name>env.TRAVIS_PULL_REQUEST</name>
      <value>!false</value>
    </property>
  </activation>
  <properties>
    <incremental.spotbugs.target>refs/heads/${env.TRAVIS_BRANCH}</incremental.spotbugs.target>
  </properties>
</profile>