Adding readme (#125)

experimental/README.md

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+# Master channel samples
+
+In this directory, you'll find samples that target Flutter's master channel.
+They may take advantage of new SDK features that haven't landed in the
+stable channel, and they may crash, lock up your machine, or otherwise fail to
+run correctly. In other words, consider everything in this directory to be an
+experiment. 
+
+We're maintaining these samples here so that folks can see some of Flutter's
+upcoming features as they evolve, and get a sense for how they'll eventually
+be used. If you'd like to run them, make sure to switch the the master channel
+first:
+
+```bash
+flutter channel master
+flutter upgrade
+```
+
+When you're done, use this command to return to the safety of the stable
+channel:
+
+```bash
+flutter channel stable
+```
+
+## Index
+
+There aren't actually any samples in here just yet. Things are coming, though!
+
+## Want more info about the repo in general?
+
+See the [README](../README.md) in the repo's root folder.