RATIS-2305. Add a security doc. (#1270)

Author: szetszwo

ratis-docs/src/site/markdown/security.md

@@ -0,0 +1,46 @@
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+
+# Security
+
+Raft is a consensus algorithm designed for fault tolerance.
+A basic assumption of Raft is that
+ 
+- the servers can trust each other.
+
+In contrast, Raft is not designed to solve the Byzantine Generals Problem,
+which assumes that some of the servers may be malicious.
+
+Apache Ratis is a Raft library
+which supports a pluggable transport.
+Applications using Ratis usually run a transport over a network.
+In such case,
+the applications must either provide secure communications between the servers
+or provide a safe network environment such as running the servers in a private network.
+Applications must not accept requests from any untrusted servers.
+
+Below are the TLS Configuration Parameters.
+Applications may use them to build `RaftServer`/`RaftClient` objects for establishing secure connections.
+
+| **Property**                            | **Description**                   |
+|:----------------------------------------|:----------------------------------|
+| `raft.grpc.tls.conf`                    | gRPC default TLS conf             |
+| `raft.grpc.server.tls.conf`             | gRPC server TLS conf              |
+| `raft.grpc.client.tls.conf`             | gRPC client TLS conf              |
+| `raft.grpc.admin.tls.conf`              | gRPC admin TLS conf               |
+| `raft.netty.dataStream.server.tls.conf` | Netty data stream server TLS conf |
+| `raft.netty.dataStream.client.tls.conf` | Netty data stream client TLS conf |