Add HACKING.md for Emacs and Vim configuration

HACKING.md

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+For contributors, we strongly suggest using the following configuration
+according to your editors.
+
+## EMACS CONFIGURATION
+
+No change needed. See [.dir-locals.el](.dir-locals.el).
+
+## VIM CONFIGURATION
+
+Please add the following script to the end of your `~/.vimrc`,
+or place in `~/.vim/after/ftplugin/c.vim` if you have other plugins.
+
+```
+" Checking if the file is cloned from https://github.com/oscarlab/graphene
+let GitUrl = system("cd ".expand('%:p:h')."; git config --get remote.origin.url 2>/dev/null")
+if GitUrl =~ "oscarlab/graphene"
+  " If so, apply the Graphene formatting rules
+  set shiftwidth=4
+  set tabstop=8
+  set expandtab
+  set textwidth=100
+  set formatoptions=tcq
+endif
+```
+
+__** Disclaimer: Due to security concerns, we do not suggest using Vim modelines or `.exrc`. **__