README.md: Need pkg-config on Linux

Change-Id: I32a2f8e2668514b6fc80163f1c567a4038c12d1c
Reviewed-on: https://dawn-review.googlesource.com/c/2340
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/README.md b/README.md
index fb465d67..e9def8c 100644
--- a/README.md
+++ b/README.md
@@ -34,9 +34,16 @@
 ## Building Dawn
 
 Dawn uses the Chromium build system and dependency management so you need to [install depot_tools] and add it to the PATH.
-Then get the source as follows:
 
-[install depots_tools]: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up
+[install depot_tools]: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up
+
+On Linux you need to have the `pkg-config` command:
+```sh
+# Install pkg-config on Ubuntu
+sudo apt-get install pkg-config
+```
+
+Then get the source as follows:
 
 ```sh
 # Clone the repo as "dawn"