qt5: Add instructions for updating and patching

pkgs/development/libraries/qt-5/README.md

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+# Qt 5 Maintainer's Notes
+
+## Minor Updates
+
+Let `$major` be the major version number, e.g. `5.9`.
+
+1. Change the version number in the `$major/fetch.sh`.
+2. Run `./maintainers/scripts/fetch-kde-qt.sh pkgs/development/qt-5/$major`
+   from the top of the Nixpkgs tree.
+
+See below if it is necessary to update any patches.
+
+## Major Updates
+
+Let `$major` be the new major version number, e.g. `5.10`.
+
+1. Copy the subdirectory from the previous major version to `$major`.
+2. Change the version number in `$major/fetch.sh`.
+3. Run `./maintainers/scripts/fetch-kde-qt.sh pkgs/development/qt-5/$major`
+   from the top of the Nixpkgs tree.
+4. Add a top-level attribute in `pkgs/top-level/all-packages.nix` for the new
+   major version.
+5. Change the `qt5` top-level attribute to point to the new major version.
+6. If the previous major version is _not_ a long-term support release,
+   remove it from Nixpkgs.
+
+See below if it is necessary to update any patches.
+
+## Patches
+
+Nixpkgs maintains several patches for Qt which cannot be submitted upstream. To
+facilitate maintenance, a fork of the upstream repository is created for each patched module:
+
+- [qtbase](https://github.com/ttuegel/qtbase)
+- [qtwebkit](https://github.com/ttuegel/qtwebkit)
+- [qttools](https://github.com/ttuegel/qttools)
+- [qtscript](https://github.com/ttuegel/qtscript)
+- [qtserialport](https://github.com/ttuegel/qtserialport)
+- [qtdeclarative](https://github.com/ttuegel/qtdeclarative)
+- [qtwebengine](https://github.com/ttuegel/qtwebengine)
+
+In each repository, the patches are contained in a branch named `nixpkgs/$major`
+for each major version. Please make a pull request to add or update any patch
+which will be maintained in Nixpkgs.
+
+The forked repository for each module is used to create a single patch in
+Nixpkgs. To recreate the patch for module `$module` (e.g. `qtbase`) at version
+`$version` (e.g. `5.9.1`) in the branch `$major` (e.g. `5.9`),
+
+1. Clone the fork for `$module` from the list above.
+2. Checkout the active branch, `git checkout nixpkgs/$major`.
+3. Compare the patched branch to the release tag,
+   `git diff v$version > $module.patch`.
+4. Copy `$module.patch` into the Nixpkgs tree.
+
+### Minor Version Updates
+
+To update module `$module` to version `$version` from an older version in the
+same branch `$major`,
+
+1. Clone the fork for `$module` from the list above.
+2. Checkout the active branch, `git checkout nixpkgs/$major`.
+3. Merge the new version into the active branch,
+   `git merge --no-ff v$version`.
+4. Fix any conflicts.
+5. Open a pull request for the changes.
+6. Follow the instructions above to recreate the module patch in Nixpkgs.
+
+### Major Version Updates
+
+To update module `$module` from `$oldversion` in branch `$oldmajor` to version
+`$version` in branch `$major`,
+
+1. Clone the fork for `$module` from the list above.
+2. Checkout a new branch for the new major version,
+   `git checkout -b nixpkgs/$major nixpkgs/$oldmajor`.
+3. Rebase the patches from `$oldversion` onto the new branch,
+   `git rebase v$oldversion --onto v$version`.
+4. Fix any conflicts.
+5. Open a pull request for the changes.
+6. Follow the instructions above to recreate the module patch in Nixpkgs.