emacsPackages: add README.md

For now it has documentation for bulk updaters.

pkgs/applications/editors/emacs/elisp-packages/README.md

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+# Emacs Lisp Packages Framework
+
+This directory includes a framework for packaging Emacs Lisp (henceforth, Elisp)
+packages to Nixpkgs.
+
+## Bulk Updating
+
+There are basically two methods for updating the Elisp package sets.
+
+### From `nix-community` Overlay
+
+The easiest one is to download and commit the package sets from [`nix-community`
+Emacs Overlay](https://github.com/nix-community/emacs-overlay). The script
+`./update-from-overlay` does all the work:
+
+- It downloads and overwrites the files `elpa-generated.nix`,
+  `elpa-devel-generated.nix`, `recipes-archive-melpa.json`,
+  `nongnu-generated.nix` and `nongnu-devel-generated.nix`;
+- Tests each package set;
+- Commits the changes.
+
+### From local update scripts
+
+We have the following scripts:
+
+- `update-package-sets`
+
+  This script accepts four possible arguments, namely `elpa`, `elpa-devel`,
+  `nongnu`, `nongnu-devel`. Each of them updates the specific package set.
+
+- `update-melpa`
+
+  It updates `recipes-archive-melpa.json` file and its corresponding
+  `emacsPackages.melpaPackages` and `emacsPackages.melpaStablePackages`
+  package sets.
+
+  After running it, and before committing its changes, it is mandatory to run
+  the following smoke test:
+
+  ```
+    env NIXPKGS_ALLOW_BROKEN=1 nix-instantiate --show-trace /../../../../../ -A "emacsPackages.melpaPackages"
+    env NIXPKGS_ALLOW_BROKEN=1 nix-instantiate --show-trace /../../../../../ -A "emacsPackages.melpaStablePackages"
+  ```
+
+  The commit should be generated with the command below:
+
+  ```
+  git commit -m "emacsPackages.{melpaPackages,melpaStablePackages}: updated at $(date --iso) (from local scripts)" -- `recipes-archive-melpa.json`
+  ```