把数据写入excel，主要是用write.table函数
用？write.table或者help(write.table)来查看帮助
以下是这个函数的一些参数：
write.table(x, file = "", append = FALSE, quote = TRUE, sep = " ",
eol = "\n", na = "NA", dec = ".", row.names = TRUE,
col.names = TRUE, qmethod = c("escape", "double"),
fileEncoding = "")
里面有个append参数，默认是FALSE
append
logical. Only relevant if file is a character string. If TRUE, the output is appended to the file. If FALSE, any existing file of the name is destroyed.
把这个参数设置成TRUE就可以把数据追加到已有的文件后面了。