Your own application code should be located mainly in the src folder. If you look at some bundles or other components of Symfony2, you can see that they all have a separate LICENSE file in the root directory of the bundle, so the right choice for your file is:
src/.../.../LICENSE
In this case, your LICENSE file LICENSE also be included if you install your kit through the composer in another project. In this case, it will be:
vendor/.../.../LICENSE
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