Document necessary PHP ini changes

[nmoinvaz]

We have a Zendesk instance that has a lot of tickets. In order to get the importer to work we had to change our php.ini with the following changes so that the importer did not time out.

memory_limit = -1
PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 262144 bytes)

max_execution_time = 0
PHP Fatal error: Maximum execution time of 300 seconds exceeded

I recommend adding to README.md the possibility of the user having to change these values if they are trying to import a large data set. I used the Error Log Monitor wordpress plugin in order to view the actual errors. But the importer didn't give any details only simply stated:

An error occurred during the import process.

[nmoinvaz]

I would also recommend adding this line from PHP max_execution_time docs:

Your web server can have other timeout configurations that may also interrupt PHP execution. Apache has a Timeout directive and IIS has a CGI timeout function. Both default to 300 seconds. See your web server documentation for specific details.

http://php.net/manual/en/info.configuration.php#ini.max-execution-time