Add README.org's for lang/cc, app/email; update other README.org's

modules/lang/cc/README.org

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+* C/C++/Obj-C
+
+I use C/C++ mostly for developing games. To that end, this module leverages the
+[[https://github.com/Sarcasm/irony-mode][irony plugin]] for auto-completion, eldoc documentation and syntax checking (with
+flycheck).
+
+Though it provides Objective-C support, I don't use it except to inspect
+generated glue code for iOS mobile apps.
+
++ Snippets ([[https://github.com/hlissner/emacs-snippets/tree/master/c-mode][c-mode]], [[https://github.com/hlissner/emacs-snippets/tree/master/c++-mode][c++-mode]])
++ File templates ([[../../feature/file-templates/templates/c-mode][c-mode]], [[../../feature/file-templates/templates/c++-mode][c++-mode]])
+
+** Installation
+~irony-server~ is required to use irony-mode's features auto-completion (among
+other things). To install it, run ~M-x irony-install-server~.
+
+This is all you need on Linux.
+
+*** MacOS
+libclang is required to build irony-server. On MacOS, the best way I know of to
+get it is:
+
+#+BEGIN_SRC sh :tangle (if (doom-system-os 'macos) "yes")
+brew install llvm  # may take a while!
+#+END_SRC