Add .md extensions so Gitiles renders READMEs.

Gitiles will render markdown README.md files, but will not render
markdown README files for some reason.

Change-Id: I1b3ab51dccf7cead0dd0041d360058b96eae9228
diff --git a/bandpass_recorder/README b/bandpass_recorder/README.md
similarity index 100%
rename from bandpass_recorder/README
rename to bandpass_recorder/README.md
diff --git a/signal_generator/README b/signal_generator/README.md
similarity index 100%
rename from signal_generator/README
rename to signal_generator/README.md
diff --git a/udp_test/README b/udp_test/README.md
similarity index 100%
rename from udp_test/README
rename to udp_test/README.md
diff --git a/utils/README b/utils/README.md
similarity index 100%
rename from utils/README
rename to utils/README.md