Add documentation for tests

mimic3benchmark/tests/README.md

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+# Tests for checking whether the benchmark datasets were generated correctly
+
+The main script is the `mimic3benchmark/tests/hash_tables.py` script which takes a directory and hashes all `.csv` files inside it recursively.
+The output is a `.pkl` file which lists all `.csv` files and their hashes.
+If this files matches with the corresponding file from `mimic3benchmark/tests/resources` then the test is successful.
+Here are a couple of examples.
+
+### Root directory
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/root -o root-final.pkl;
+diff root-final.pkl mimic3benchmark/tests/resources/root-final.pkl;
+```
+
+### In-hospital mortality
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/in-hospital-mortality -o ihm.pkl;
+diff ihm.pkl mimic3benchmark/tests/resources/ihm.pkl;
+```
+
+### Decompensation
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/decompensation -o decomp.pkl;
+diff decomp.pkl mimic3benchmark/tests/resources/decomp.pkl;
+```
+
+### Length-of-stay
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/length-of-stay -o los.pkl;
+diff los.pkl mimic3benchmark/tests/resources/los.pkl;
+```
+
+### Phenotyping
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/phenotyping -o pheno.pkl;
+diff pheno.pkl mimic3benchmark/tests/resources/pheno.pkl;
+```
+
+### Multitasking
+```bash
+python -um mimic3benchmark.tests.hash_tables -d data/multitask -o multitasking.pkl;
+diff multitasking.pkl mimic3benchmark/tests/resources/multitasking.pkl;
+```