UPGRADE-2.5.md: Yaml component change in 2.5

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

We had a very hard to find bug when I was trying to upgrading symfony from 2.4 to 2.5, I think it's worth mentioning in the UPGRADE file. I understand we shouldn't have had duplicated keys in yaml but that was it...
BTW maybe we should raise an error in cases like this?

PS I figured I shouldn't include the PR template with table here because it's just a UPGRADE-* file change, I will add it if you say