Initial background markdown document

BACKGROUND.md

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+# Activity-based Model
+
+Guide to background and modelling assumptions made.
+
+## Assumptions
+
+### Geographies
+Since the SPC currently uses 2011 OA11CD and MSOA11CD codes, 2011 boundaries will be used.
+
+#### Locations
+- Homes: OA11CD centroids
+- Work: exact location data
+- Education: exact location, travel time for a given mode is used to determine the feasible zone.
+- Shop (food):
+- Shop (other):
+- Visit:
+
+### Primary location modelling
+- TODO