Add first draft of constraints document

First attempt to address #221.

constraints/constraints.md

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+# Constraints
+
+The following document details the reasons why we do some of the things we do.
+
+## Our constraints
+
+As part of a big company that delivers scientific content, there are some constraints that define the work that we do.
+
+Particularly, the solutions that we deliver:
+
+* must be accessible,
+* must be Technically Robust and Performant,
+* must be Legally Robust,
+* will reflect Management & Org Structure,
+* are bound by our Infrastructure,
+* need to be appropriately Secure, and
+* need to reflect the Company's Stature.
+
+As a result of these constraints, the solutions that we provide are:
+
+* [Progressively enhanced](../practices/house-style.md#progressive-enhancement-and-browser-support)
+* Compliant with [WCAG 2.0 level AA](../practices/house-style.md#ccessibility)