We have a function, that renders the list item:

renderPosts() {
    return _.map(this.props.posts, post => {
        return (
            <li className="list-group-item" key={post.id}>
                {post.title}
            </li>
        )
    })
}

Now let's add a Link:

import {Link} from 'react-router-dom';
...
    renderPosts() {
        return _.map(this.props.posts, post => {
            return (
                <li className="list-group-item" key={post.id}>
                    <Link to={`/posts/${post.id}`}>
                        {post.title}
                    </Link>
                </li>
            )
        })
    }