Sync USAGE.md.

USAGE.md

@@ -1,35 +1,44 @@
 # How to write ebooks with Markdown
 
 * Organize articles in directories, each directory is a chapter.
+* Order of chapters is hard-coded in file `convert.sh`, so that we can avoid
+  complex/unnecessary way to order them.
 * Chapter title is defined in file `_title.md` under chapter folder.
+* Chapter summary is defined in file `_summary.md` under chapter folder.
+* Addition links stored in file `_links.md` will be appended to chapter.
 
 ```
-- 1-introduction/
-    |- _title.md
+- chapter_name_1/
+    |- _title.md: Chapter title. REQUIRED
+    |- _links.md: Addition links, will be appended to chapter. OPTIONAL
+    |- _summary.md: Summary text of this chapter, will be displayed under
+                    chapter title. OPTIONAL
     |- what_is_iredmail.md
     |- why_choose_iredmail.md
-    |- price.md
-- 2-faq/
-- 3-install/
+- chapter_name_2/
+- chapter_name_3/
 ...
 - html/css/markdown.css
 - html/images/
 ```
 
-* Articles will be ordered automatically, if you want to force the order,
-  prepend a digit number and `-` in article files. e.g.
+* Articles will be ordered automatically, if you want to specify the order,
+  prepend a digit number and `-` in article files, script `convert.sh` will
+  remove this prefix during converting Markdown source file to HTML file. e.g.
 
 ```
-- 1-introduction/
+- chapter_name_x/
     |- _title.md
     |- 1-what_is_iredmail.md
     |- 2-why_choose_iredmail.md
     |- 3-price.md
 ```
 
-* Convert all Markdown files to HTML static files. Script `convert.sh` will
-also generate index file `html/index.html` (which includes all articles with
-valid internal links).
+* Run script `convert.sh` to convert Markdown files to HTML static files.
+
+    * It will generate index file `html/index.html` which includes all articles
+      with relative links to HTML files.
+    * it also generates README.md file used by bitbucket.org as index page.
 
 ```bash
 $ bash convert.sh