[SitemapBundle] Make use of a sitemapindex and sub sitemaps

[denbatte]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	yes
Deprecations?	no
Fixed tickets	#4

WHAT
This feature generates one central sitemap.xml file at your document root to define a sub sitemap for each language available.
You can now have up to 50000 urls for each language you define.
As a bonus, you only have to register the sitemap index (/sitemap.xml) at the search engines to register all available languages.

HOW
In order to upgrade, you need a new route in your app/config/routes.yml.

KunstmaanSitemapBundle_sitemapIndex:
    resource: "@KunstmaanSitemapBundle/Controller/SitemapController.php"
    type:     annotation

TODO

• add a note in UPGRADE-3.2.md file

[krispypen]

@roderik 👍

[roderik]

@krispypen we need to defer this to 3.2, it features a bc break because it adds a new route. Currently i'm using master as 3.1.1-dev. Let's keep this one open for a while.

[roderik]

@denbatte please create an UPGRADE-3.2.md doc and add this info