Merge pull request #139 from nf-core/docs_update

Further clarification on `--max-cpus`

Author: apeltzer

docs/usage.md

@@ -237,7 +237,7 @@ Use to set a top-limit for the default time requirement for each process.
 Should be a string in the format integer-unit. eg. `--max_time '2.h'`. If not specified, will be taken from the configuration in the `-profile` flag.
 
 ### `--max_cpus`
-Use to set a top-limit for the default CPU requirement for each process.
+Use to set a top-limit for the default CPU requirement for each **process**. This is not the maximum number of CPUs that can be used for the whole pipeline, but the maximum number of CPUs each program can use for each program submission (known as a process). Do not set this higher than what is available on your workstation or computing node can provide. If you're unsure, ask your local IT administrator for details on compute node capabilities! 
 Should be a string in the format integer-unit. eg. `--max_cpus 1`. If not specified, will be taken from the configuration in the `-profile` flag.
 
 ### `--email`