Move VERSIONS description to file itself (#5434)

stdlib/VERSIONS

@@ -1,3 +1,19 @@
+# The structure of this file is as follows:
+# - Blank lines and lines starting with `#` are ignored.
+# - Lines contain the name of a top-level module, followed by a colon,
+#   a space, and a version range (for example: `symbol: 2.7-3.9`).
+#
+# Version ranges may be of the form "X.Y-A.B" or "X.Y-". The
+# first form means that a module was introduced in version X.Y and last
+# available in version A.B. The second form means that the module was
+# introduced in version X.Y and is still available in the latest
+# version of Python.
+#
+# Python versions before 2.7 are ignored, so any module that was already
+# present in 2.7 will have "2.7" as its minimum version. Version ranges
+# for unsupported versions of Python 3 (currently 3.5 and lower) are
+# generally accurate but we do not guarantee their correctness.
+
 __future__: 2.7-
 __main__: 2.7-
 _ast: 2.7-