Update instructions to include how to satisfy gFlags prerequisite on a mac
diff --git a/doc/command_line_tool.md b/doc/command_line_tool.md
index 6337aca..a373cbe 100644
--- a/doc/command_line_tool.md
+++ b/doc/command_line_tool.md
@@ -41,12 +41,16 @@
git submodule update --init
```
-You also need to have the gflags library installed on your system. On Linux
-systems, gflags can be installed with the following command:
-
+You also need to have the gflags library installed on your system. gflags can be
+installed with the following command:
+Linux:
```
sudo apt-get install libgflags-dev
```
+Mac systems with Homebrew:
+```
+brew install gflags
+```
Once the prerequisites are satisfied, you can build the command line tool with
the command:
