Create the layer README on create-layer

When using `configuration-layer/create-layer`, the `README.org` file is
not created from the template. This change corrects this.

Close #3259

core/core-configuration-layer.el

@@ -215,8 +215,9 @@ layer directory."
                        "this layer already exists.") name))
      (t
       (make-directory layer-dir t)
-      (configuration-layer//copy-template name "extensions" layer-dir)
-      (configuration-layer//copy-template name "packages" layer-dir)
+      (configuration-layer//copy-template name "extensions.el" layer-dir)
+      (configuration-layer//copy-template name "packages.el" layer-dir)
+      (configuration-layer//copy-template name "README.org" layer-dir)
       (message "Configuration layer \"%s\" successfully created." name)))))
 
 (defun configuration-layer/make-layer (layer)
@@ -408,9 +409,9 @@ If LAYER_DIR is nil, the private directory is used."
   (let ((src (concat configuration-layer-template-directory
                      (format "%s.template" template)))
         (dest (if layer-dir
-                  (concat layer-dir "/" (format "%s.el" template))
+                  (concat layer-dir "/" (format "%s" template))
                 (concat (configuration-layer//get-private-layer-dir name)
-                        (format "%s.el" template)))))
+                        (format "%s" template)))))
     (copy-file src dest)
     (find-file dest)
     (save-excursion

doc/CONTRIBUTE.org

@@ -56,7 +56,7 @@ layers in. For example a layer for a language would go in the
 It is recommended to join a =README.org= file with your layer:
   - ideally this file should document the packages of your layer as well as the
     key bindings associated with them
-  - a template is provided in =~/.emacs.d/core/templates/layer-README.template=,
+  - a template is provided in =~/.emacs.d/core/templates/README.org.template=,
     use it as much as possible
   - another good practice is to start from the =README.org= of an existing layer
   - if a logo exists for the layer you can add it at the top of the =README.org=

doc/CONVENTIONS.org

@@ -328,7 +328,7 @@ as an help command.
 
 * Writing documentation
 Spacemacs provides an example layer =README.org= file in
-=~/.emacs.d/core/templates/layer-README.template=.
+=~/.emacs.d/core/templates/README.org.template=.
 
 ** Spacing in documentation
 Spacemacs tries to keep the documentation consistent between all layers by