Add deprecation plan for URLs with quoted paths

docs/howtos/index.md

@@ -22,6 +22,7 @@ These cookbooks teach you how to solve specific tasks:
 .. toctree::
    :maxdepth: 1
 
+   migrations/Deprecation-of-quoted-paths-in-URLs
    migrations/Deprecation-of-Optional-fold-and-Optional-build
    migrations/Deprecation-of-old-union-literal-syntax
    migrations/Deprecation-of-old-Optional-literal-syntax

docs/howtos/migrations/Deprecation-of-quoted-paths-in-URLs.md

@@ -0,0 +1,56 @@
+# Deprecation of quoted paths in URLs
+
+> Migrate your code to remove URLs with quoted path components
+
+On July 1, 2019 the [language standard changed](https://github.com/dhall-lang/dhall-lang/pull/604) to allow RFC3986-compliant URLs.  This section describes what changed and how to migrate your code.
+
+## Changes
+
+Before this change, if you wanted to use special characters in a URL path component, you had to quote it:
+
+```dhall
+http://example.com/"foo<bar.dhall"
+```
+
+After this change, the correct way to format this URL is via percent-encoding:
+
+```dhall
+http://example.com/foo%3Cbar.dhall
+```
+
+All RFC 3986 style URLs are permitted, except that we do not permit the characters `(`, `)` or `,` in URLs to prevent confusion with surrounding Dhall code.
+
+## Phases
+
+The old URL quoted-path syntax will be phased out in three steps:
+
+* Phase 1 - Allow standard RFC 3986 URLs
+
+  * Standard version: [9.0.0](https://github.com/dhall-lang/dhall-lang/releases/tag/v9.0.0)
+  * `dhall` version: [1.25.0](https://github.com/dhall-lang/dhall-haskell/releases/tag/1.25.0)
+
+  The first phase is backwards compatible.  The only difference is that RFC 3986 standard URLs are now permitted, including percent-encoding.
+
+* Phase 2 - `dhall format` now rewrites quoted URLs as percent-encoded URLs
+
+  * `dhall` version: [1.26](https://github.com/dhall-lang/dhall-haskell/releases/tag/1.26.0)
+
+  This change does not change language behaviour; but it means that code using deprecated syntax will be automatically formatted to use the new syntax.
+
+* Phase 3 - quoted URLs are no longer allowed.
+
+  This change is backwards incompatible by removing support for the old quoted-path-component syntax for URLs, breaking all code that still uses the old syntax.
+
+  * Standard version: [17.0.0](https://github.com/dhall-lang/dhall-lang/releases/tag/v17.0.0)
+
+## Migration
+
+* Phase 1 - No action required
+
+* Phase 2 - Automatically migrate your code
+
+   During this phase `dhall format` will automatically migrate the old quoted-path URL syntax to use percent-encoding instead to ease the migration process.  This is a safe and behavior-preserving transformation.
+
+* Phase 3 - Your code breaks if you haven't migrated
+
+  During Phase 3 the deprecation cycle is complete and if you haven't migrated then your code will fail to parse.