build: add description of prerequisite to build

Let's check two more packages before building f2fs-tools.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/README b/README
index c7b4a31..cbb492e 100644
--- a/README
+++ b/README
@@ -9,6 +9,8 @@
 
 Your should install the following packages.
  - libuuid-devel or uuid-dev
+ - pkg-config
+ - autoconf
 
 Initial compilation
 -------------------