;site: symlink the main README.md, render it as part of the site

The top-level README.md is usually seen when browsing github or the
cloned source - not on the website - but now we render it as part of
the website to facilitate previewing it locally (with make
site-watch).

As a side effect of symlinking it into site/, it might override site/'s
README when browsing that directory on github.

[ci skip]

Shake.hs

@@ -224,8 +224,7 @@ main = do
         ,"index"
         ,"intro"
         ,"release-notes"
-        -- some copied from elsewhere
-        -- ,"README"
+        ,"README"
         ,"CONTRIBUTING"
         ]
 

site/README

@@ -1,4 +1,10 @@
+This is the README for the site/ directory.
+
+(Confusingly, README.md from the parent directory is also symlinked
+here, to get it rendered as part of the website, and it might take
+precedence when viewing this dir on github.)
+
 Source files for the hledger.org website. 
-To render this, run ./Shake website. 
+To generate the website, run ./Shake website. 
 This will add the web manuals' markdown to this directory, 
 render all markdown as html, and copy static files, to site/_site/.

site/README.md

@@ -0,0 +1 @@
+../README.md