docs/README.md: to render nicer pages on github

... as previously the README.cmake would be picked and put at the bottom
of the docs page there and it wasn't very representative!

docs/README.md

@@ -0,0 +1,12 @@
+![curl logo](https://cdn.rawgit.com/curl/curl-www/master/logo/curl-logo.svg)
+
+# Documentation
+
+You'll find a mix of various documentation in this directory and
+subdirectories, using several different formats. Some of them are not ideally
+for reading directly in your browser.
+
+If you rather see the rendered version of the documentation, check out the
+curl web site's [documentation section](https://curl.haxx.se/docs/) for
+general curl stuff or the [libcurl section](https://curl.haxx.se/libcurl/) for
+libcurl related documentation.