Add Setting Filetype With Modelines as a vim til

vim/setting-filetype-with-modelines.md

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+# Setting Filetype With Modelines
+
+Vim and various plugins generally use known file extensions to determine the
+filetype of a file. This is important because it is how Vim decides which
+filetype-specific settings to enable, such as syntax highlighting.
+
+If I am editing a file such as `build.boot`, Vim is not going to know that
+its filetype should be set to `clojure`. The `build.boot` file is full of
+clojure code though, so I'm losing out on syntax highlighting and so forth.
+I can settle for manually setting the filetype to clojure (e.g. `:set
+ft=clojure`) each time I open up the file.
+
+Or I can use a modeline setting. By including a comment at the top or
+bottom of the file specifying the filetype setting, I can ensure that each
+time I go to edit the file, the appropriate filetype will be set.
+
+That modeline comment will look something like:
+
+```clojure
+; vim: set ft=clojure:
+```
+
+See `:h modeline` for more details.
+
+h/t Brian Dunn