Add explanations for this directory.
diff --git a/testdata/README.txt b/testdata/README.txt
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+This directory contains various collections of files used for test
+data. It currently contains the following collections:
+
+largefiles/
+ This contains some really large files; most are used for
+ benchmarking various aspects of Expat's performance.
diff --git a/testdata/largefiles/README.txt b/testdata/largefiles/README.txt
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+This directory contains some really large test files, mostly used to
+benchmark various aspects of Expat's performance.
+
+(As files are added, they should be described here, including what
+benchmark program they're intended to be used with and what that
+resulting measurements tell us.)
