More polish, hide unfinished chapters/levels

levels/unused/remotes-add

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+title = Adding a remote
+cards = checkout
+
+[description]
+
+Let's work together with others! Your friend has their own repo at the URL `../friend` - you can add it using
+
+    git remote add [name] [URL]
+
+where `[name]` is an arbitrary, short name you pick for the remote.
+
+When you've done that, you can get all commits from that remote using
+
+    git pull friend
+
+There's a letter for you!
+
+[setup]
+
+git remote remove friend
+
+[setup friend]
+
+echo "I'm really committed to our friendship! <3" > love_letter
+git add .
+git commit -m "Write a letter"
+
+[win]
+
+# Add a remote that points to ../friend.
+git remote -v | grep '../friend'
+# Pull from the remote.
+git show HEAD:love_letter | grep committed