Create warning for appending mode 'a' operations

[tassilucas]

Add specific warning for files in appending mode since gcsfs ignores mode='a' as stated in issue #533.

[martindurant]

Thanks for the suggestion - it is reasonable.

I wonder whether someone would like to try to implement append mode via compose , i.e., write new data to a temporary file and then join the parts.
For small initial files (< block_size), they can simply be read and re-written.