Add a readme for the data/ directory, but ignore its content (my data)

.gitignore

@@ -1,2 +1,3 @@
 curl_key.txt
-curl_key_guest.txt
+curl_key_guest.txt
+data/

data/README.md

@@ -0,0 +1,22 @@
+data
+==
+
+This is where your data goes.  Don't forget to (manually) replicate my directory structure, or update the Python scripts accordingly.  And replace my user name (gully) with yours.
+
+
+
+```bash
+$ tree -d gully/
+gully/
+├── exercise
+│   ├── HR
+│   ├── logs
+│   └── tcx
+├── intraday
+│   ├── HR
+│   ├── sleep
+│   └── steps
+└── weight
+    ├── logs
+    └── ts
+```