Add parameters policy document

PARAMETERS.md

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+Parameter Policy
+====================
+
+Goal: document parameter handling approach
+
+Parameter Parsing
+-------------------
+
+Ideally, the string received from the user input is parsed into the the
+Python type it most closely matches. That means that `'True'` parses
+to `True`, `'1'` to `1`, `'20000.0'` to `20000.0`, and so on. It is then
+up to the upstream package to perform type and range validation on the
+converted object and provide the appropriate error messages.
+
+However, due to issues with the interface, the user's input has to be massaged
+into the correct format. For instance, places where we expect a boolean
+string display `1.0` as a default value instead of `True`. Thus, the value
+parsing logic must be capable of parsing `'1.0'`, `'1'`, or `'True'` to `True`.
+There are other issues such as values that should only be integers (e.g. age)
+are displayed as floating point values. Once we display more accurate
+default values, we should remove this functionality in order to simplify the
+codebase. Note that there will always be some element of string massaging
+since PolicyBrain users tend to be less technical and thus, may not realize
+the significance of entering `True` versus `true` or `TRUE`.
+
+
+Parameter Deprecation
+---------------------
+
+Our primary goal is to preserve the results and relevant information for all
+runs. The goal of allowing the user to edit and re-submit previous runs is a
+lower priority than the first goal. The underlying modeling packages change
+overtime which means that parameter names change, their underlying definitions
+change, they are split in to multiple components, and some are removed.
+PolicyBrain only allows the user to view and submit parameters that are
+supported by the current version of the underlying package. Thus, when a user
+tries to access a run submitted on an older version of the underlying package
+via the edit parameters page, some of the parameters may not exist on the page
+anymore. The PolicyBrain policy in this situation is to allow the user to view
+and edit the parameters that still exist and for the parameters that do not,
+display an error message at the top of the page for each missing parameter that
+says:
+
+    Field {parameter name} has been deprecated. Refer to the Tax-Caclulator
+    documentation for a sensible replacement.