docs: Start a changelog. Format & tooling is a WIP.

Author: timoxley

CHANGELOG.md

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+## [5.2.1](https://github.com/streamr-dev/streamr-client/compare/v5.1.0...v5.2.1) (2021-03-29)
+
+This release fixes a subtle but serious memleak in all previous 5.x
+versions. Upgrading immediately is highly recommended.
+
+### Bug Fixes
+
+* **dataunions:** Withdraw amount can be a string ([bccebcb](https://github.com/streamr-dev/streamr-client/commit/bccebcb580e91f55a68a0aae864dcc48cf370bfa))
+* **keyexchange:** Remove bad call to cancelTask. ([cb576fd](https://github.com/streamr-dev/streamr-client/commit/cb576fdccbbf2600011c89a151cb15a3870a258a))
+* **pipeline:** Fix memleak in pipeline. ([86fcb83](https://github.com/streamr-dev/streamr-client/commit/86fcb833d74df267ad538fc37cf76c4f95c9462c))
+* **pushqueue:** Empty pending promises array in cleanup, prevents memleak. ([1e4892c](https://github.com/streamr-dev/streamr-client/commit/1e4892ccabd6853c59648fc025bc8be1fd630e55))
+* **pushqueue:** Transform was doing something unusual. ([27dbb05](https://github.com/streamr-dev/streamr-client/commit/27dbb05612a2d838e4ce399fb643d6ca00c1af74))
+* **subscribe:** Clean up type errors. ([1841741](https://github.com/streamr-dev/streamr-client/commit/184174127835a3d11160acbe4804b621e4480a86))
+* **util:** Clean up dangling Defers in LimitAsyncFnByKey. ([1eaa55a](https://github.com/streamr-dev/streamr-client/commit/1eaa55a9b51f44e055148ba80fa51e1d63fe2a77))
+* **util/defer:** Rejig exposed functions so resolve/reject can be gc'ed. ([2638f2b](https://github.com/streamr-dev/streamr-client/commit/2638f2b164455b6c45f3c6e9d3d6b297669ebc7a))
+* **validator:** Only keep a small message validator cache. ([de7b689](https://github.com/streamr-dev/streamr-client/commit/de7b68953dd52f8fbdf4bee4dff2ba6b3bd02545))