docs: move INSTALL, AUTHORS, HACKING and TODO to docs/

And with this, cleanup README to be shorter and mention the new source
code home.

docs/HACKING

@@ -0,0 +1,13 @@
+
+libssh2 source code style guide:
+
+ - 4 level indent
+ - spaces-only (no tabs)
+ - open braces on the if/for line:
+
+     if (banana) {
+         go_nuts();
+     }
+
+ - keep source lines shorter than 80 columns
+ - See libssh2-style.el for how to achieve this within Emacs