Update some READMEs under dev/. (#15712)

diff --git a/dev/README.md b/dev/README.md
new file mode 100644
index 0000000..afe8f26
--- /dev/null
+++ b/dev/README.md
@@ -0,0 +1,4 @@
+This directory contains tools and resources that the Flutter team uses
+during development of the framework. The tools in this directory
+should not be necessary for developing Flutter applications, though of
+course they may be interesting if you are curious.