Add CHANGELOG entries for 0.18, 0.19, and 0.20. (#123)

Author: sunfishcode

CHANGELOG.md

@@ -1,3 +1,23 @@
+# origin 0.20
+
+## Changes
+
+The `origin-program`, `origin-thread`, and `origin-signal` features are now
+combined into a single `take-charge` feature.
+
+# origin 0.19
+
+## Changes
+
+`current_tls_addr` now has a `module` parameter instead of hard-coding the
+module to be 1.
+
+# origin 0.18
+
+## Changes
+
+The `set_thread_id` feature is now enabled unconditionally.
+
 # origin 0.17
 
 ## Changes