Add readme for build/args directory.

build/args/README.txt

Index: build/args/README.txt
diff --git a/build/args/README.txt b/build/args/README.txt
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+This directory is here to hold .gni files that contain sets of GN build
+arguments for given configurations.
+
+(Currently this directory is empty because we removed the only thing here, but
+this has come up several times so I'm confident we'll need this again. If this
+directory is still empty by 2017, feel free to delete it. --Brett)
+
+Some projects or bots may have build configurations with specific combinations
+of flags. Rather than making a new global flag for your specific project and
+adding it all over the build to each arg it should affect, you can add a .gni
+file here with the variables.
+
+For example, for project foo you may put in build/args/foo.gni:
+
+  target_os = "android"
+  use_pulseaudio = false
+  use_ozone = true
+  system_libdir = "foo"
+
+Users wanting to build this configuration would run:
+
+  $ gn args out/mybuild
+
+And add the following line to their args for that build directory:
+
+  import("//build/args/foo.gni")
+  # You can set any other args here like normal.
+  is_component_build = false
+
+This way everybody can agree on a set of flags for a project, and their builds
+stay in sync as the flags in foo.gni are modified.