Update the documentation on Running CaffeOnSpark on YARN

[xiaoyongzhu]

I am trying to run CaffeOnSpark on YARN based on this wiki (https://github.com/yahoo/CaffeOnSpark/wiki/GetStarted_yarn), but there are a few problems there.

It would be better if the documentation can mention:

1. Update the path in data/lenet_memory_train_test.prototxt to the actual path
2. Change the mnist_train_lmdb and mnist_train_test folder to writeable (per https://github.com/yahoo/CaffeOnSpark/issues/95).

I would love to contribute to the wiki but don't know how, since I don't see the wiki page in the code repository:( So raise an issue here in case someone else can edit the wiki.

[junshi15]

@anfeng Do you know how to update the wiki?