Add dummy Android.mk to document its absence

The Android.mk provided by the upstream fmtlib repository is for use
with the NDK rather than the platform build, so we have replaced it
with an Android.bp. This change adds back a dummy Android.mk with a
comment documenting this so as to not overly confuse future
maintainers.

Test: None, documentation only
Change-Id: I242a8238acda88c62d3612b33bb59b387efc5885
diff --git a/Android.mk b/Android.mk
new file mode 100644
index 0000000..0660b8a
--- /dev/null
+++ b/Android.mk
@@ -0,0 +1,4 @@
+# The Android.mk provided by the upstream fmtlib repository is for use with the
+# NDK and is not appropriate for the platform build. When integrating changes
+# from upstream, omit their Android.mk and leave the existing Android.bp
+# instead.