Added philosophy.md

docs/philosophy.md

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+> This document contains philosophical notes about nu.
+
+# Bare Words
+
+In Nu, bare words work the same way they do in most shells.
+
+In most shells, bare words serve two purposes:
+
+```
+$ ls
+# ^^ the name of a command
+$ cat Cargo.toml
+#     ^^^^^^^^^^ a string
+```
+
+Nu adopts this shell idiom.
+
+Consequences:
+
+-   Bare words cannot also refer to variables. Variable names are prefixed with `$`.
+-   Bare words, in almost all contexts, cannot be keywords.
+-   Numbers and operators aren't bare words.
+
+# One Screen
+
+The utility of a command's output drops off extremely rapidly after a screenful of content.
+
+By default, Nu prefers to present output that can fit into a screen rather than more complete output that spans many screens.
+
+For example, this is the rationale for `ls` returning a flat table containing the files in the current directory, rather than presenting a tree of data by default.