First shot of a documentation of the new PSR-4 class loader.
Adjustments from comments be @WouterJ
Minor corrections.
Adjustments from comments by @bicpi
Moved versionadded block to the top.
feature #3686 Documentation of the new PSR-4 class loader. (derrabus)

This PR was merged into the master branch.

Discussion
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Documentation of the new PSR-4 class loader.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#10100)
| Applies to    | 2.5+
| Fixed tickets | #3655

Hello docs team,

this is a first draft of a documentation for the PSR-4 class loaded I've contributed. As this is my first contribution to the documentation, any assistance would be appreciated. :-)

Commits
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cb2be4a Moved versionadded block to the top.
16fead4 Adjustments from comments by @bicpi
a05da41 Minor corrections.
6f2a1a3 Adjustments from comments be @WouterJ
17166bd First shot of a documentation of the new PSR-4 class loader.