Add a skill for writing mutations

skills/write-mutations/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: Write mutations
+description: Trigger when asked to write mutations
+---
+
+Annotate tests with one or more mutations (`MutationCase`s) that will cause the test to fail.
+
+Example:
+
+```fs
+[<Fact>]
+[<MutationCase("calc-operator-mixup", "Calculator/Calculator.fs", 4, "value + 1", "value - 1")>]
+member _.AddOne_increments() =
+    Assert.Equal(42, Calculator.addOne 41)
+```
+
+In this example,
+
+* `calc-operator-mixup` is the mutation name,
+* `Calculator/Calculator.fs` is the path to the production code (relative to the repository root),
+* `4` is the number of the line to mutate,
+* `value + 1` is the string to find, and
+* `value - 1` is the string to replace it with.
+
+Verify your work with `mutannot --run <mutation id 1> <mutation id 2> <...>`.