chore: add a changelog to shared & sdk

Author: Stebalien

sdk/CHANGELOG.md

@@ -0,0 +1,5 @@
+# Changelog
+
+## 0.5.0 - 2022-04-11
+
+Upgrades the SDK to fvm_shared 0.5.0. This release includes a significant breaking change to exit codes.

shared/CHANGELOG.md

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+# Changelog
+
+## 0.5.0 - 2022-04-11
+
+- Enforce maximum big-int size to match lotus.
+- Make signature properties public.
+- Major error type refactor.
+
+The largest change here is a major error type refactor.
+
+1. It's now a u32 with a set of pre-defined values instead of an enum.
+2. The error codes have been reworked according to the FVM spec.
+
+Both of these changes were made to better support user-defined actors.