backend-common: Add changeset about SHA based caching in URL Reader

.changeset/khaki-icons-trade.md

@@ -0,0 +1,22 @@
+---
+'@backstage/backend-common': patch
+---
+
+1. URL Reader's `readTree` method now returns a `sha` in the response along with the blob. The SHA belongs to the latest commit on the target. `readTree` also takes an optional `sha` in its options and throws a `NotModifiedError` if the SHA matches with the latest commit SHA on the url's target. This can be used in building a cache when working with URL Reader.
+
+An example -
+
+```ts
+const response = await reader.readTree(
+  'https://github.com/backstage/backstage',
+);
+
+const commitSha = response.sha;
+
+// Will throw a new NotModifiedError (exported from @backstage/backstage-common)
+await reader.readTree('https://github.com/backstage/backstage', {
+  sha: commitSha,
+});
+```
+
+2. URL Reader's readTree method can now detect the default branch. So, `url:https://github.com/org/repo/tree/master` can be replaced with `url:https://github.com/org/repo` in places like `backstage.io/techdocs-ref`.