Add README for informal specs directory.

docs/language/informal/README.md

Index: docs/language/informal/README.md
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+This directory contains "informal specifications".
+
+In order to move faster and get better feedback, we implement and iterate on
+language changes before the full official specification has been written. Still,
+the implementers need *something* to go on.
+
+For that, the language team writes "informal specifications". These are
+intended to be precise enough for a good faith implementer to correctly
+understand the syntax and semantics of the language, but without all of the
+laborious detail of the complete specification.
+
+Once the feature has been implemented, tested, and we are confident in it, the
+language team will write the real text in the language specification and the
+document here becomes deprecated and no longer canonical.
+
+Until then, while the feature is in progress, a live pull request for the
+informal spec is the source of truth for the feature.