Adding documentation about beans.xml. Closes #815.

@Turini There is no difference to place under `META-INF` or `WEB-INF`. So what you think to describe here only about `WEB-INF`? My question is because `META-INF` must be located under `classpath`, and users can confuse because applications can have another `META-INF` under root folder. And another point is that this `META-INF` must placed under `/src/main/resources` for users who uses Maven, but for users who uses Eclipse without Maven by default uses `/src`. Another IDEs have another project structure. Since there is many cases to place this file properly under `classpath`, I think that is more simple to describe only `WEB-INF`. Makes sense?