[FAB-4864] Doc update for max file descriptors

This change set adds a warning to the README to notify developers that
they may need to increase max file descriptors to run the unit tests.
With all of the new tests, the number of required file descriptors has
increased, and OSX's default is only 256, thus the recent failure.

Change-Id: I0a8c0d9483e2a8dca5122f00640f0665e5a51f40
Signed-off-by: Keith Smith <bksmith@us.ibm.com>

README.md

@@ -16,7 +16,7 @@ See [User's Guide for Fabric CA](https://hyperledger-fabric-ca.readthedocs.io) f
 ## Contribution guidelines
 
 You are welcome to contribute to Fabric CA!
-   
+
 The following are guidelines to follow when contributing:
 
 1. See the general information about [contributing to fabric](http://hyperledger-fabric.readthedocs.io/en/latest/CONTRIBUTING.html).
@@ -32,9 +32,28 @@ The following are guidelines to follow when contributing:
    # cdr
    # make unit-tests
    ```
-   
+
    The test coverage for each package must be 75% or greater.  If this fails due to insufficient test coverage, then you can run `gencov` to get a coverage report to see what code is not being tested.   Once you have added additional test cases, you can run `go test -cover` in the appropriate package to see the current coverage level.
 
+   WARNING: Running the unit-tests may fail due to too many open file descriptors.
+   Depending on where the failure occurs, the error message may not be obvious and may only say something similar to "unable to open database file".
+   Depending on the settings on your host, you may need to increase the maximum number of open file descriptors.
+   For example, the OSX default per-process maximum number of open file descriptors is 256.
+   You may issue the following command to display your current setting:
+
+   ```
+   # ulimit -n
+   256
+   ```
+
+   And the following command will increase this setting to 65536:
+
+   ```
+   # ulimit -n 65536
+   ```
+
+   Please note that this change is only temporary. To make it permanent, you will need to consult the documentation for your host operating system.
+
 ## Package overview
 
 1. **cmd/fabric-ca-server** contains the main for the fabric-ca-server command.
@@ -64,8 +83,8 @@ Run `go tool pprof -h` to view the options supported by the pprof tool. For more
 
 See [FVT tests](scripts/fvt/README.md) for information on functional verification test cases.
 
-    
- 
+
+
 
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