[wip] emacsPackages: add README.md

For now it has documentation for bulk updaters.

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

@@ -0,0 +1,58 @@
+# 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 scripts
+
+We have the following scripts:
+
+- `lib-update-scripts.sh`
+
+  This file serves as a library, containing useful functions to deal with bulk updates.
+  It can be `source`'d in both interactive and batch environments.
+
+  The following functions are provided:
+
+  - `download_change`
+  - `test_packageset`
+  - `commit_change`
+
+- `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`
+  ```