Add links to emacs modes into doc

[a4z]

There are at least 2 emacs djinni.el modes available

• https://github.com/danielmartin/djinni-mode
• https://github.com/finalpatch/djinni-mode/

differences as described by @finalpatch

Other than minor differences in syntax highlighting:

His mode provides a shortcut to run the compilation (by invoking .run_djinni.sh under the same dir by default) and highlight errors (no sure how well that works).  my mode doesn't have this.

His indentation is more simplistic (increase indent on { and decrease on }) and is a lot slower (takes several seconds to re-indent a large djinni file).  my mode has more sophisticated indentation (eg.  can indent parameters on different lines) and works much faster.

My mode provides Imenu support for quickly jump to interface and records