Add a file to track per-crate semver status.

doc/semver_status.md

+# Semver tracking
+
+This is a helpful file that we use for checking which crates will have
+breaking or nonbreaking API changes in the next release of Arti.
+
+For each crate, please write "BREAKING" if there is an API change that counts
+as breaking in semver, and "MODIFIED" if there is a backward-compatible API
+change.
+
+You can change from MODIFIED to BREAKING, but never from BREAKING to
+MODIFIED.
+
+You don't need to list details; this isn't the changelog.
+
+Don't document other changes in this file.
+
+We can delete older sections here after we bump the releases.
+
+
+## Since Arti 0.0.1
+
+tor-proto: BREAKING
+tor-client: MODIFIED
+tor-persist: BREAKING if `testing` feature is enabled.
+tor-dirclient: MODIFIED