[doc-only] Add a short toolshed/README.md (#2102)

Author: rwgk

toolshed/README.md

@@ -0,0 +1,10 @@
+# Shared Toolshed
+
+This directory is a shared scratchpad for scripts that are useful
+intermittently, such as environment bootstrap helpers, ad-hoc reproducers, and
+glue for manual workflows. These tools do not warrant CI coverage, unit tests,
+or the rest of the production-code apparatus.
+
+Entries may bitrot between uses. Keeping them in a shared, versioned location
+gives the next person who needs the same workflow a better starting point than
+starting from scratch.