[doc] add examples of headerNode/footerNode usage to readme

[dodikk]

[doc] add examples of headerNode/footerNode usage to readme

[dodikk]

Here is a snippet for adding an avatar and user name to the headerView of the MessageNode

    internal static func addAvatar(
        withUrl strAvatarUrl: String?,
        userName: String,
        toCell result: MessageNode,
        isIncoming isIncomingMessage: Bool,
        parentVC: ChatRoomView,
        imageLoader: ASImageDownloaderProtocol,
        imageCache: ASImageCacheProtocol)
    {
        let isMessageHasAvatar = (nil != strAvatarUrl)
        let maybeAvatarUrl = strAvatarUrl.flatMap(URL.init(string:))
        
        let isShowingIcon =
            isMessageHasAvatar &&
            (isIncomingMessage || self._shouldShowMyAvatar) // HPV-2455 - do not show My Avatar in a chat
        
        guard isShowingIcon
        else
        {
            return
        }

        
        // ==
        //
        let avatarAndUserNameBuilder: ASDisplayNodeViewBlock =
        {
            () -> UIView in
            
            let nib =
                UINib(
                    nibName: "VHChatMessageHeaderView",
                    bundle: Bundle.main)
            
            let nibObjects =
                nib.instantiate(
                    withOwner: nil,
                    options: nil)
            
            let userInfoView = nibObjects.first as! VHChatMessageHeaderView
            
            // inject data
            //
            userInfoView.avatarView?.image = nil
            if let avatarUrl = maybeAvatarUrl
            {
                userInfoView.avatarView?.setImage(mediaURL: avatarUrl)
            }
            userInfoView.usernameLabel?.text = userName
            
            return userInfoView
        }
        
        let avatarAndUserName = ASDisplayNode(viewBlock: avatarAndUserNameBuilder)
        avatarAndUserName.style.preferredSize = CGSize(width: 375, height: 24)
        
        result.headerNode = avatarAndUserName
    }

P.S. I could not make it render unless avatarAndUserName.style.preferredSize was specified