add fish

fish.md

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+# fish
+
+Read [fish](https://wiki.archlinux.org/index.php/fish).
+
+## Configuration
+
+Web interface:
+
+```sh
+fish_config
+```
+
+Command completion, require [man page](https://wiki.archlinux.org/index.php/Man_page):
+
+```sh
+fish_update_completions
+```
+
+## Tips and tricks
+
+Disable greeting:
+
+```sh
+set -U fish_greeting
+```