slog-handler-guide/README.md: stub

Add a file that will ultimately contain a guide to writing slog handlers.

This CL just creates the file so we can make a shortlink to it.

Change-Id: Ic31e62c6d92a480ecb7ba03cf052ca375f5abb94
Reviewed-on: https://go-review.googlesource.com/c/example/+/509735
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

slog-handler-guide/README.md

diff --git a/slog-handler-guide/README.md b/slog-handler-guide/README.md
new file mode 100644
index 0000000..d4a1d2e
--- /dev/null
+++ b/slog-handler-guide/README.md
@@ -0,0 +1,3 @@
+# A Guide to Writing `slog` Handlers
+
+This file will contain a guide to writing a slog handler.