Added eval operator

pkg/yqlib/doc/operators/headers/eval.md

@@ -0,0 +1,7 @@
+# Eval
+
+Use `eval` to dynamically process an expression - for instance from an environment variable.
+
+`eval` takes a single argument, and evaluates that as a `yq` expression. Any valid expression can be used, beit a path `.a.b.c | select(. == "cat")`, or an update `.a.b.c = "gogo"`.
+
+Tip: This can be useful way parameterise complex scripts.