[DOCS]: Describe TCP mode (Lopez mode) [ci skip] (#2045)

Closes #1937

docs/tcp_mode.md

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+# TCP mode
+
+Puma also could be used as a TCP server to process incoming TCP
+connections.
+
+
+## Configuration
+
+TCP mode can be enabled with CLI option `--tcp-mode`:
+
+```
+$ puma --tcp-mode
+```
+
+Default ip and port to listen to are `0.0.0.0` and `9292`. You can configure
+them with `--port` and `--bind` options:
+
+```
+$ puma --tcp-mode --bind tcp://127.0.0.1:9293
+$ puma --tcp-mode --port 9293
+```
+
+TCP mode could be set with a configuration file as well with `tcp_mode`
+and `tcp_mode!` methods:
+
+```
+# config/puma.rb
+tcp_mode
+```
+
+When Puma starts in the TCP mode it prints the corresponding message:
+
+```
+puma --tcp-mode
+Puma starting in single mode...
+...
+* Mode: Lopez Express (tcp)
+```
+
+
+## How to declare an application
+
+An application to process TCP connections should be declared as a
+callable object which accepts `env` and `socket` arguments.
+
+`env` argument is a Hash with following structure:
+
+```ruby
+{ "thread" => {}, "REMOTE_ADDR" => "127.0.0.1:51133", "log" => "#<Proc:0x000..." }
+```
+
+It consists of:
+* `thread` - a Hash for each thread in the thread pool that could be
+  used to store information between requests
+* `REMOTE_ADDR` - a client ip address
+* `log` - a proc object to write something down
+
+`log` object could be used this way:
+
+```ruby
+env['log'].call('message to log')
+#> 19/Oct/2019 20:28:53 - 127.0.0.1:51266 - message to log
+```
+
+
+## Example of an application
+
+Let's look at an example of a simple application which just echoes
+incoming string:
+
+```ruby
+# config/puma.rb
+app do |env, socket|
+  s = socket.gets
+  socket.puts "Echo #{s}"
+end
+```
+
+We can easily access the TCP server with `telnet` command and receive an
+echo:
+
+```shell
+telnet 0.0.0.0 9293
+Trying 0.0.0.0...
+Connected to 0.0.0.0.
+Escape character is '^]'.
+sssss
+Echo sssss
+^CConnection closed by foreign host.
+```
+
+
+## Socket management
+
+After the application finishes, Puma closes the socket. In order to
+prevent this, the application should set `env['detach'] = true`.