irmp: introduce sigrok specific README file

Introduce a separate README-sigrok.txt file, to leave the upstream
project's README.txt file as is. Mention that libsigrokdecode only
contains a subset of the full IRMP project source code.

diff --git a/irmp/README-sigrok.txt b/irmp/README-sigrok.txt
new file mode 100644 (file)
index 0000000..03ff472
--- /dev/null
+++ b/irmp/README-sigrok.txt
@@ -0,0 +1,3 @@
+This directory contains a subset of the IRMP project source code. Then
+extends it to allow sigrok protocol decoders to use the IRMP backend.
+See README.txt for information on the IRMP project itself.