Fix README.md so that relative links from manual become absolute.

Previously they'd be broken links when viewed on GitHub or Hackage.
So we add the base URL for the pandoc manual.

tools/update-readme.lua

@@ -8,15 +8,21 @@ f:close()
 result = {}
 
 function Div(elem)
-    local ident = elem.identifier or ""
-    local get = function(el)
-                    if el.identifier == ident then
-                        result = el
-                    end
-                end
-    if ident == 'input-formats' or ident == 'output-formats' then
-      pandoc.walk_block(pandoc.Div(mdoc.blocks), { Div = get })
-      return result
+  local ident = elem.identifier or ""
+  local fixrel = function(el)
+    if el.target:match("^#") then
+      el.target = "https://pandoc.org/MANUAL.html" .. el.target
     end
+    return el
+  end
+  local get = function(el)
+    if el.identifier == ident then
+      result = pandoc.walk_block(el, { Link = fixrel  })
+    end
+  end
+  if ident == 'input-formats' or ident == 'output-formats' then
+    pandoc.walk_block(pandoc.Div(mdoc.blocks), { Div = get })
+    return result
+  end
 end