minor symfony#20856 [Routing] Mention minor BC break about UrlGenerator & query strings (ogizanagi)

This PR was merged into the 3.2 branch.

Discussion
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[Routing] Mention minor BC break about UrlGenerator & query strings

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no, mention one
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

---

Refs:

- https://github.com/javiereguiluz/EasyAdminBundle/pull/1421#issuecomment-266201441
- symfony#19639
- http://php.net/manual/en/function.http-build-query.php
- https://www.ietf.org/rfc/rfc3986.txt

Commits
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9e73887 [Routing] Mention minor BC break about UrlGenerator & query strings

Author: fabpot

UPGRADE-3.2.md

@@ -112,6 +112,12 @@ HttpKernel
    Surrogate-Capability: symfony="ESI/1.0"
    ```
 
+Router
+------
+
+ * `UrlGenerator` now generates URLs in compliance with [`RFC 3986`](https://www.ietf.org/rfc/rfc3986.txt),
+    which means spaces will be percent encoded (%20) inside query strings.
+
 Serializer
 ----------