Update documentation

(execute the script for help):

```bash

usage: dl_dep.sh [-h] [-n <NAPLUGIN>] [-c <CLUSTER>]

usage: dl_dep.sh [-h] [-l] [-n <NAPLUGIN>] [-c <CLUSTER>] [-d <DEPENDENCY>] 

                    source_path

optional arguments:

        -h, --help              shows this help message and exits

        -l, --list-dependencies

                                list dependencies available for download

        -n <NAPLUGIN>, --na <NAPLUGIN>

                                network layer that is used for communication. Valid: {bmi,ofi,all}

                                defaults to 'all'

        -c <CLUSTER>, --cluster <CLUSTER>

                                additional configurations for specific compute clusters

                                supported clusters: {mogon1,fh2}

                                supported clusters: {mogon1,mogon2,fh2}

        -d <DEPENDENCY>, --dependency <DEPENDENCY>

                                download a specific dependency. If unspecified 

                                all dependencies are built and installed.

```

- Now use the install script to compile them and install them to the desired directory. You can choose the according

na_plugin (execute the script for help):

```bash

usage: compile_dep.sh [-h] [-n <NAPLUGIN>] [-c <CLUSTER>] [-j <COMPILE_CORES>]

usage: compile_dep.sh [-h] [-l] [-n <NAPLUGIN>] [-c <CLUSTER>] [-d <DEPENDENCY>] [-j <COMPILE_CORES>]

                      source_path install_path

optional arguments:

    -h, --help  shows this help message and exits

    -l, --list-dependencies

                list dependencies available for building and installation

    -n <NAPLUGIN>, --na <NAPLUGIN>

                network layer that is used for communication. Valid: {bmi,ofi,all}

                defaults to 'all'

    -c <CLUSTER>, --cluster <CLUSTER>

                additional configurations for specific compute clusters

                supported clusters: {mogon1,mogon2,fh2}

    -d <DEPENDENCY>, --dependency <DEPENDENCY>

                build and install a specific dependency. If unspecified 

                all dependencies are built and installed.

    -j <COMPILE_CORES>, --compilecores <COMPILE_CORES>

                number of cores that are used to compile the depdencies

                number of cores that are used to compile the dependencies

                defaults to number of available cores

    -t, --test  Perform libraries tests.

```

## Compile GekkoFS

an MPI application use the `{mpirun, mpiexec} -x` argument.

### Logging

To enable logging the following environment variables are used:

GKFS_PRELOAD_LOG_PATH="<path/to/file>" to set the path to the logging file of the client library.

GKFS_DAEMON_LOG_PATH="<path/to/file>" to set the path to the logging file of the daemon.

GKFS_LOG_LEVEL={off,critical,err,warn,info,debug,trace} to set the trace level verbosity.

Numbers from 0-6 may also be used where as 0 is off and 6 represents trace.

The following environment variables can be used to enable logging in the client

library: `LIBGKFS_LOG=<module>` and `LIBGKFS_LOG_OUTPUT=<path/to/file>` to

configure the output module and set the path to the log file of the client

library.

The following modules are available:

 - `none`: don't print any messages

 - `syscalls`: Trace system calls: print the name of each system call, its

   arguments, and its return value. All system calls are printed after being

   executed save for those that may not return, such as `execve()`,

   `execve_at()`, `exit()`, and `exit_group()`. This module will only be

   available if the client library is built in `Debug` mode.

 - `syscalls_at_entry`: Trace system calls: print the name of each system call

   and its arguments. All system calls are printed before being executed and

   therefore their return values are not available in the log. This module will

   only be available if the client library is built in `Debug` mode.

 - `info`: Print information messages.

 - `critical`: Print critical errors.

 - `errors`: Print errors.

 - `warnings`: Print warnings.

 - `mercury`: Print Mercury messages.

 - `debug`: Print debug messages.  This module will only be available if the

   client library is built in `Debug` mode.

 - `most`: All previous options combined except `syscalls_at_entry`. This

   module will only be available if the client library is built in `Debug`

   mode.

 - `all`: All previous options combined.

 - `help`: Print a help message and exit.

Additionally, setting the `LIBGKFS_LOG_OUTPUT_TRUNC` environment variable with

a value different from `0` will instruct the logging subsystem to truncate 

the file used for logging, rather than append to it.

For the daemon, the `GKFS_DAEMON_LOG_PATH=<path/to/file>` environment variable 

can be provided to set the path to the log file, and the log module can be 

selected with the `GKFS_LOG_LEVEL={off,critical,err,warn,info,debug,trace}`

environment variable.

### Acknowledgment