Implement CLI command for tagging

how_to/Change_20.md

@@ -0,0 +1,41 @@
+- tag: Implement CLI command
+
+Now that we have branching history we have some OIDs we need to keep track of.
+Assume we have two branches (continuing from the example we had for `checkout`):
+```
+o-----o-----o-----o-----@-----@-----@
+^                  \                ^
+first commit        ----$-----$     6c9f80a187ba39b4...
+                              ^
+                              d8d43b0e3a21df0c...
+```
+
+If we want to switch back and forth between the two "branches" with `checkout`,
+we need to remember both OIDs, which are quite long.
+
+To make our lives easier, let's implement a command to attach a name to an OID.
+Then we'll be able to refer to the OID by that name.
+
+The end result will look like this:
+```
+$ # Make some changes
+...
+$ ugit commit
+d8d43b0e3a21df0c845e185d08be8e4028787069
+$ ugit tag my-cool-commit d8d43b0e3a21df0c845e185d08be8e4028787069
+$ # Make more changes
+...
+$ ugit commit
+e549f09bbd08a8a888110b07982952e17e8c9669
+
+$ ugit checkout my-cool-commit
+        or
+$ ugit checkout d8d43b0e3a21df0c845e185d08be8e4028787069
+```
+
+The last two commands are equivalent, because "my-cool-commit" is a tag that
+points to d8d43b0e3a21df0c845e185d08be8e4028787069.
+
+We will implement this in a few steps. The first step is to create a CLI
+commmand that call the relevant command in the base module. The base module does
+nothing at this stage.