Add change 10 instructions

how_to/Change_10.md

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+- write-tree: Hash the files
+
+Instead of only printing the file name, let's put all files in the object
+database. For now we'll print their OID and their name.
+
+Notice that instead of getting one OID to represent a directory we now get a
+separate OID for each file, which isn't very useful. Plus, note that the names
+of the files aren't stored in the object database, they are just printed and
+then the information is discarded.
+
+So at this stage `write-tree` isn't useful (it just saves a bunch of files as
+blobs) but the next change will fix it.