doc/tutorial/basics: add basics tutorial
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+[TOC](Readme.md) [Prev](fieldcomp.md) [Next](coalesce.md)
+
+# Conditional Fields
+
+Field comprehensions can also be used to
+add a single field conditionally.
+
+Converting the resulting configuration to JSON results in an error
+as `justification` is required yet no concrete value is given.
+
+
+<!-- CUE editor -->
+```
+price: float
+
+// Require a justification if price is too high
+justification: string if price > 100
+
+price: 200
+```
+
+<!-- result -->
+```
+price: 200
+justification: string
+```
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