Introduce submodule

[KengoTODA]

Problem

• We put both of source and generated files (HTML, CSS, images, etc.) to the same branch, so our diff in PR becomes really dirty

Solution in this pull request

This PR adds a build directory, which works as a Git submodule.
Later we can add more PR to delete generated files from master branch.

I also pushed gh-pages branch to this repository, which has the same content with master branch. We can use gh-pages this page to host static pages instead of master branch.

refs: https://github.com/shiftyp/submodule-gh-pages-example

[jsotuyod]

This approach doesn't seem to be working. I introduced a change on the index of gh-pages' index.html

However, I don't see this when accessing the page at https://spotbugs.github.io/

Are we missing something? Maybe the gh-pages approach doesn't work for *.github.io repositories...

[KengoTODA]

Oh, yes, you are right...

User and Organization Pages that have this type of repository name are only published from the master branch.
https://help.github.com/articles/configuring-a-publishing-source-for-github-pages/

Then we need to use special branch not to host site but to manage sources. I will recreate similar pull request. Thanks for your review!