[Filter] add usage notes for makedeps, rewrite and include-md (#132)

filters/hugo_makedeps.lua

@@ -92,6 +92,12 @@ images and local links to Markdown files. For each such link, this process will
 (recursively, via breadth-first search).
 
 
+Usage: This filter is intended to be used with individual files that are placed directly in
+the working directory.
+Examples:
+    pandoc -L hugo_makedeps.lua -t markdown readme.md
+
+
 Caveats:
 (a) All referenced Markdown files must have UNIQUE NAMES.
 (b) References to the top index page (landing page) are (presumably) not working.

filters/hugo_rewritelinks.lua

@@ -31,6 +31,14 @@ Thus this filter needs to perform for any link in any Markdown file:
 -   replace  `[Y](readme.md)`         with  `[Y]({{< ref "/" >}})` (might work with Hugo)
 
 
+Usage: This filter is intended to be used with individual files that are placed either directly in
+the working directory or in a subdirectory.
+Examples:
+    pandoc -L hugo_rewritelinks.lua -t markdown readme.md
+    pandoc -L hugo_rewritelinks.lua -t markdown test/readme.md
+    pandoc -L hugo_rewritelinks.lua -t markdown test/subdir/leaf/foo.md
+
+
 Caveats:
 (a) All referenced Markdown files must have UNIQUE NAMES.
 (b) References to the top index page (landing page) are (presumably) not working.

filters/include-mdfiles.lua

@@ -15,6 +15,13 @@ for each link to local Markdown file in start document:
             - return block list to replace current Para
 
 
+Usage: This filter is intended to be used with individual files that are placed directly in
+the working directory.
+Examples:
+    pandoc -L include-mdfiles.lua -t markdown readme.md
+    pandoc -L include-mdfiles.lua -t markdown summary.md
+
+
 Caveats:
 The same file cannot be included more than once to avoid potential endless recursion.
 ]]--