readme: copy walking-through example to tests

It can be useful to have a simple example with go-connector usage with
detailed comments. Walking-through example is a single example that left
in a README. The goal of example is to be an entrypoint for new users
and to be an initial snippet suitable for playing with Go connector.

Add a description with steps how-to run example manually.

Author: ligurio

README.md

@@ -21,7 +21,7 @@ faster than other packages according to public benchmarks.
 * [Installation](#installation)
 * [Hello World](#hello-world)
 * [API reference](#api-reference)
-* [Walking\-through example in Go](#walking-through-example-in-go)
+* [Walking\-through example](#walking-through-example)
 * [Help](#help)
 * [Usage](#usage)
 * [Schema](#schema)
@@ -108,7 +108,7 @@ The source file for error-handling tools is
 which has structure definitions and constants whose names are equivalent to names
 of errors that the Tarantool server returns.
 
-## Walking-through example in Go
+## Walking-through example
 
 We can now have a closer look at the `example.go` program and make some observations
 about what it does.

example_walk_through_test.go

@@ -0,0 +1,47 @@
+// Run Tarantool instance before example execution:
+// Terminal 1:
+// $ TT_LISTEN=3013 TT_WORK_DIR=$(mktemp -d -t 'tarantool.XXX') tarantool config.lua
+//
+// Terminal 2:
+// $ go test -v example_test.go
+package tarantool_test
+
+// The line "`github.com/tarantool/go-tarantool`" in the
+// `import(...)` section brings in all Tarantool-related functions and
+// structures.
+import (
+	"fmt"
+	"github.com/tarantool/go-tarantool"
+)
+
+func Example_walkThrough() {
+	// The line beginning with "Opts :=" sets up the options for
+	// Connect(). In this example, there is only one thing in the structure, a user
+	// name. The structure can also contain:
+	//  - Pass (password),
+	//  - Timeout (maximum number of milliseconds to wait before giving up),
+	//  - Reconnect (number of seconds to wait before retrying if a
+	//  connection fails),
+	//  - MaxReconnect (maximum number of times to retry).
+	opts := tarantool.Opts{User: "guest"}
+	// The line containing "tarantool.Connect" is essential for
+	// beginning any session. There are two parameters:
+	//  - a string with host:port format or path to a UNIX socket, and
+	//  - the optional structure that was set up earlier.
+	// The err structure will be nil if there is no error,
+	// otherwise it will have a description which can be retrieved with err.Error().
+	conn, err := tarantool.Connect("127.0.0.1:3301", opts)
+	if err != nil {
+		fmt.Println("Connection refused:", err)
+	}
+	// The Insert request, like almost all requests, is preceded by
+	// "conn." which is the name of the object that was returned by Connect().
+	// There are two parameters:
+	//  - a space number (it could just as easily have been a space name), and
+	//  - a tuple.
+	resp, err := conn.Insert(999, []interface{}{99999, "BB"})
+	if err != nil {
+		fmt.Println("Error", err)
+		fmt.Println("Code", resp.Code)
+	}
+}