Add HCA data as submodule. Update README (txt ➔ md).

.gitmodules

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+[submodule "custom-data/raw/hca-fhir-importer"]
+	path = custom-data/raw/hca-fhir-importer
+	url = https://github.com/smart-on-fhir/hca-fhir-importer.git

README.md

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+SMART on FHIR Test Data Generator
+=================================
+
+This generator uses the data files in the `data` directory to generate
+FHIR test data.
+The files in the data directory are tab-delimited tables that can be edited and extended with new data as needed.
+
+All the Python scripts are in the `bin` directory, and should be run from
+that directory.
+This repository contains git submodules to handle 3-rd party sample data, make sure you checkout using the _--recursive_ option.
+
+The main script for general use is `generate.py`, the other files in `bin` 
+are basically modules supporting generate.py.
+The file `testdata.py` contains most of the constant declarations and utility functions that drive the system configuration.
+The file `fhir.py` contains all the FHIR formatting code. 
+
+From the 'bin' directory, run:
+
+    python generate.py --help
+
+To get a general help message.
+
+To generate the test data files in the 'generated-data' directory:
+
+    python generate.py --write-fhir ../generated-data 
+
+And a `summary.txt` file can be added to `generated-data` by running:
+
+    python generate.py --summary > ../generated-data/summary.txt
+
+A good way to look at a single patient, with patient ID, PID, is:
+
+    python generate.py --summary PID