README.md: Misc updates (#388)

First and foremost, try and explain that EditorConfig is bundled
with Emacs and that it's both "the same yet different".
Also, mention that ideally major modes should be changed
instead of adding stuff to `editorconfig-indentation-alist`.

Author: monnier

README.md

@@ -9,6 +9,18 @@ This is an [EditorConfig][] plugin for [Emacs][].
 
 ## Getting Started
 
+If you're using Emacs≥30, then support for EditorConfig is bundled
+directly with Emacs and all you need to do is:
+
+```emacs-lisp
+(editorconfig-mode 1)
+```
+
+The code that comes with Emacs derives from the code in this separate
+package but has a few differences, mostly because it uses hooks that are new
+in Emacs-30.  In most cases it behaves in the same way, and the two codes
+are (still) in the process of being synchronized.
+
 ### package.el
 
 This package is available from [MELPA][], [MELPA Stable][] and [NonGNU ELPA][].
@@ -141,7 +153,12 @@ sorry to say, it cannot be perfect. Especially it is difficult to support
 brand-new major-modes.
 Please feel free to submit issue or pull-request for such major-mode!
 
-Supported major-modes and their indentation configs are defined in the variable
+In most cases it is better to modify the major mode itself
+either to use a "standard" indentation size variable name
+like `<MODE>-indent-offset` or to set `editorconfig-indent-size-vars`.
+
+Those major modes which cannot yet follow this convention need to have
+their indentation configs defined in the variable
 `editorconfig-indentation-alist`.
 
 ### Not work at all for FOO-mode!