Add readme files to some directories.

Change-Id: I2a750c8e46b68dc4870e2dadd2ed3429008e1562
Reviewed-on: https://pdfium-review.googlesource.com/13551
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/.gitignore b/.gitignore
index b1ea7a5..9611bce 100644
--- a/.gitignore
+++ b/.gitignore
@@ -37,3 +37,5 @@
 *.user
 *.vcxproj
 *.xcodeproj
+*~
+