add README

README.md

@@ -1 +1,21 @@
-# lls
+# lls
+
+lls is lightweight ls. Using lls, you can get a list of files in a directory that contains a large number of files.
+
+## How?
+
+You allocate a buffer for the size of the directory, and then call the getdents64 system call directly.
+
+The larger the size of the directory, the more memory you will need. By running `ls -dl`, you will know how much memory to allocate in advance.
+
+See [this article](http://be-n.com/spw/you-can-list-a-million-files-in-a-directory-but-not-with-ls.html) for more information.
+
+## Usage
+
+```sh
+$ lls > output.txt
+
+$ lls / > output.txt
+```
+
+If you want to reduce the memory usage, you can specify the `-buf-size` option. If the size you specify is smaller than the actual size needed, you will not get the full file list.