minor symfony#21202 [Cache] Add changelog for 3.2 (nicolas-grekas)

This PR was merged into the 3.2 branch.

Discussion
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[Cache] Add changelog for 3.2

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

Commits
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9aaa8ac [Cache] Add changelog for 3.2

Author: fabpot

src/Symfony/Component/Cache/CHANGELOG.md

@@ -0,0 +1,18 @@
+CHANGELOG
+=========
+
+3.2.0
+-----
+
+ * added TagAwareAdapter for tags-based invalidation
+ * added PdoAdapter with PDO and Doctrine DBAL support
+ * added PhpArrayAdapter and PhpFilesAdapter for OPcache-backed shared memory storage (PHP 7+ only)
+ * added NullAdapter
+
+3.1.0
+-----
+
+ * added the component with strict PSR-6 implementations
+ * added ApcuAdapter, ArrayAdapter, FilesystemAdapter and RedisAdapter
+ * added AbstractAdapter, ChainAdapter and ProxyAdapter
+ * added DoctrineAdapter and DoctrineProvider for bidirectional interoperability with Doctrine Cache