bootstrap4中使用fontawesome5.6.3

先下载fontawesome5.6.3，选择free for web，下载完解压，丢在资源目录下

<form action="">
<div class="form-group">
    <div class="input-group">
        <input 
        type="text" 
        class="form-control"
        placeholder="请输入用户名" />
        <div class="input-group-append">
            <div class="input-group-text">
                <i class="fa fa-user" aria-hidden="true"></i>
            </div> 
        </div>
    </div>
</div>
<div class="formgroup">
    <div class="input-group">
        <input 
        type="password" 
        class="form-control"
        placeholder="请输入密码" />
        <div class="input-group-append">
            <div class="input-group-text">
                <i class="fa fa-lock" aria-hidden="true"></i>
            </div>
        </div>
    </div>
</div>
</form>