First take on README.

README.md

@@ -1,2 +1,44 @@
-Concatenate file(s) to standard output.
+[![License (3-Clause BSD)](https://img.shields.io/badge/license-BSD%203--Clause-blue.svg?style=flat-square)](http://opensource.org/licenses/BSD-3-Clause)
 
+# dcat: Concatenate file(s) to standard output.
+
+A **cat** command-line implemenation in [Dart](https://dart.dev/), loosely based on the [Write command-line apps sample code](https://dart.dev/tutorials/server/cmdline).
+
+## Command-Line Usage
+
+```sh
+dcat --help
+```
+```
+Usage: dcat [OPTION]... [FILE]...
+Concatenate FILE(s) to standard output.
+
+With no FILE, or when FILE is -, read standard input.
+
+  -b, --number-nonblank    number nonempty output lines, overrides -n
+  -E, --show-ends          display $ at end of each line
+  -h, --help               display this help and exit
+  -n, --number             number all output lines
+  -T, --show-tabs          display TAB characters as ^I
+  -s, --squeeze-blank      suppress repeated empty output lines
+      --version            output version information and exit
+
+Examples:
+  dcat f - g  Output f's contents, then standard input, then g's contents.
+  dcat        Copy standard input to standard output.
+  ```
+ # Compile Application
+  
+## Linux
+```sh
+dart compile exe -o bin/dcat bin/dcat.dart
+```
+
+## Windows
+```cmd
+dart compile.exe bin/dcat.dart
+```
+
+# Differences from GNU cat
+  - No binary file support.
+  - Line numbers are printed as `X:` where `X` is the line number.