Add stability notice and reference unstable/lens in main docs

lens/main.scrbl

@@ -1,5 +1,7 @@
 #lang scribble/manual
 
+@(require "private/doc-util/main.rkt")
+
 @title[#:style '(toc)]{Lenses}
 
 @defmodule[lens]
@@ -14,8 +16,11 @@ representation of getters and setters in object-oriented languages.
 
 source code: @url["https://github.com/jackfirth/lens"]
 
+@stability-notice[unstable/lens]
+
 @local-table-of-contents[]
 
+@secref{unstable/lens}
 @include-section["private/base/main.scrbl"]
 @include-section["private/compound/main.scrbl"]
 @include-section["private/list/main.scrbl"]

lens/private/doc-util/main.rkt

@@ -3,6 +3,7 @@ syntax/parse/define
 "deflenses.rkt"
 "include-sections.rkt"
 "lenses-examples.rkt"
+"stability-notice.rkt"
 for-label
   lens
   unstable/lens

lens/private/doc-util/stability-notice.rkt

@@ -0,0 +1,10 @@
+#lang sweet-exp racket/base
+
+provide stability-notice
+
+require scribble/manual
+
+
+(define-syntax-rule (stability-notice id)
+  (list "This library is stable, backwards compatibility will be maintained. For experimental features see "
+                 (racketmodname id) "."))