Mention appending with >>.

Fixes #151.

README.md

@@ -51,7 +51,7 @@ Notes:
 
 - Know how to read documentation with `man` (for the inquisitive, `man man` lists the section numbers, e.g. 1 is "regular" commands, 5 is files/conventions, and 8 are for administration). Find man pages with `apropos`. Know that some commands are not executables, but Bash builtins, and that you can get help on them with `help` and `help -d`.
 
-- Learn about redirection of output and input using `>` and `<` and pipes using `|`. Learn about stdout and stderr.
+- Learn about redirection of output and input using `>` and `<` and pipes using `|`. Know `>` overwrites the output file and `>>` appends. Learn about stdout and stderr.
 
 - Learn about file glob expansion with `*` (and perhaps `?` and `{`...`}`) and quoting and the difference between double `"` and single `'` quotes. (See more on variable expansion below.)