Documentation: Add EmacsConfiguration.md for emacs editor support

Emacs takes a little work to get working with Serenity.

Documentation/EmacsConfiguration.md

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+# Emacs Project Configuration
+
+Emacs can be configured with `lsp-mode` and `clangd` to work well.
+
+### clangd
+
+The official clangd extension can be used for C++ comprehension. You
+can use the following `.clangd` file placed in the project root:
+
+```yaml
+CompileFlags:
+  CompilationDatabase: Build/i686
+  Add:
+    - "-D__serenity__"
+    - "-UNO_TLS"
+    - "-I/path/to/serenity/Toolchain/Local/i686/i686-pc-serenity/include/c++/11.2.0"
+    - "-I/path/to/serenity/Toolchain/Local/i686/i686-pc-serenity/include/c++/11.2.0/i686-pc-serenity"
+```
+
+You will need to change `/path/to/serenity` and change `11.2.0` to
+whatever your GCC toolchain version at the time is.
+
+Run cmake (`Meta/serenity.sh run` or similar) at least once for this
+to work, as it will generate the `Build/i686/compile_commands.json`
+that is needed by `clangd`.
+
+### lsp-mode
+
+```lisp
+(use-package lsp-mode
+  :hook ((c++-mode) . lsp-deferred)
+  :commands lsp
+  :config
+  (setq lsp-clients-clangd-args '("-j=4" "-background-index" "--log=error" "--clang-tidy" "--enable-config"))
+  ;; Optionally, set the location of clangd -- See notes below for options.
+  (setq lsp-clangd-binary-path "/usr/bin/clangd"))
+
+```
+
+### clangd
+
+There are a few different ways to specify which clangd to use:
+
+- By default, without configuration `lsp-mode` will try to find and use your system `clangd`. This is the easiest solution, but your system clangd might be out of date.
+- You can manually specify any `clangd` binary with `lsp-clangd-binary-path`, as shown in the use-package example above.
+- You can have `lsp-mode` manage your `clangd` installation with emacs' `lsp-install-server`. This will install a `clangd` binary for you.
+- You can build the LLVM toolchain, including `clangd`, from Serenity's repository. This is an advanced option that is not currently documented.
+
+### clang-format
+
+There are multiple packages to handle auto formatting with
+`clang-format`, within emacs. Choose what works best for your setup:
+
+- [format-all-mode](https://github.com/lassik/emacs-format-all-the-code)
+- [clang-format-plus](https://github.com/SavchenkoValeriy/emacs-clang-format-plus)
+
+```