feat: updates fish_setup, README.md

fish/theme-t31m/fish_greeting.fish

@@ -0,0 +1,13 @@
+set -g CMD_DURATION 0
+
+function dim
+    set_color -o 4f4f4f
+end
+
+function off
+    set_color -o normal
+end
+
+function fish_greeting
+	echo "Welcome to FiSH "(dim)(uname -nrs)(off)
+end