Add notes on how to sample PyMC models

doc/sampling_pymc_models.md

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+## Sampling PyMC models
+
+Steps to sample PyMC models: 
+
+1. load the model json data from the respctive zipfile as a Python dictionary. 
+
+For example, for the GLM binomial model:
+
+```python
+import json
+import zipfile
+
+# Load zipfile
+zfile = zipfile.ZipFile(r"posterior_database/data/data/GLM_Binomial_data.json.zip", 'r')
+data = json.loads(zfile.read("GLM_Binomial_data.json"))
+```
+
+2. Call the model defining function, with the data dictionary and, in a `with` context, call `pm.sample`:
+
+```python
+import pymc as pm
+
+from posterior_database.models.pymc.GLM_Binomial_model import model
+
+# Call the function that creates the PyMC model and sample it
+with model(data):
+    trace = pm.sample()
+
+print(pm.stats.summary(trace))
+```