Add change 22 instructions

how_to/Change_22.md

@@ -0,0 +1,28 @@
+- tag: Create the tag ref
+
+After we've implemented refs in the previous change, it's time to create a ref
+when the user creates a tag.
+
+`create_tag` now calls update_ref with the tag name to actually create the tag.
+
+For namespacing purposes, we'll put all tags under *refs/tags/*. That is, if the
+user creates *my-cool-commit* tag, we'll create *refs/tags/my-cool-commit* ref
+to point to the desired OID.
+
+Then we'll update *data.py* to handle this "namespaced" ref. Since we can't have
+a / in the file name, we'll create directories for it. Now if a ref
+*refs/tags/sometag* is created, it will be placed under *.ugit/refs/tags* in a
+file named *sometag*.
+
+To verify that this code works, you can run:
+```
+$ ugit tag test
+```
+
+And make sure that the tag points to HEAD:
+```
+$ cat .ugit/refs/tags/test
+$ cat .ugit/HEAD
+```
+
+The last two commands should give the same output.