Rework levels, and add some new ones

levels/blob-create/description

@@ -9,3 +9,7 @@ Let's create some blobs! To do that, create a file with the desired content, and
 The flag -w means "write", and tells Git to actually write the new blob to the disk.
 
 Create three new blobs!
+
+Tip: you can also use a command like this to create a blob in a single line:
+
+    $ echo "awesome content" | git hash-object -w --stdin