rename .Rmd to .Rmarkdown and add the build script

exampleSite/R/build.R

@@ -0,0 +1,6 @@
+file.copy('content/_index.markdown', '../README.md', overwrite = TRUE)
+system("sed -i '' -e '1,6d' -e '34,38d' ../README.md")
+cat(
+  '[![Screenshot](https://github.com/yihui/hugo-xmin/raw/master/images/screenshot.png)](https://xmin.yihui.name)\n',
+  file = '../README.md', append = TRUE
+)