contrib/plugins: init with mTLS store example

This reverts commit 2b42901ec7.
Reason for revert: the work required has been achieved.

Change-Id: I917b2f87e436a4e34bec0b788634d949b0236c62

contrib/plugins/mtls-http-binary-cache-store.md

@@ -0,0 +1,13 @@
+R"(
+
+**Store URL format**: `https+mtls://...`
+
+This store allows a binary cache to be accessed via the HTTPS
+protocol with mutual TLS mandated.
+
+Two parameters can be passed to the query string:
+
+- `tls-certificate`, a path to the TLS client certificate (optional)
+- `tls-private-key`, a path to the TLS private key backing the client certificate (required)
+
+)"