docs: Update README.md

# cargo

# Cargo

Cargo is a HPC data staging service that runs alongside applications helping 

them to transfer data in parallel between local and shared storage tiers.

> This software was partially supported by the EuroHPC-funded project ADMIRE

>  (Project ID: 956748, https://www.admire-eurohpc.eu).

## Dependencies

- Margo (tested with v0.9.8)

- Argobots (tested with v1.1rc2)

- Mercury (tested with v2.1.0rc3)

- Thallium (tested with 5daa9a909d1309620ed874832cf8075644727a8a)

- libboost (tested with 1.71.0)

- MPI (tested with OpenMPI 4.0.3)

## Building Cargo

## Installation

### Building Cargo and its dependencies with Spack

Cargo and its dependencies can be built using

[Spack](https://github.com/spack/spack). If you already have Spack, make sure

you have the latest release. If you use a clone of the Spack `develop`

branch, be sure to pull the latest changes.

#### Install Spack

If you haven't already, install Spack with the following commands:

```shell

$ git clone -c feature.manyFiles=true https://github.com/spack/spack

```

This will create a directory called `spack` in your machine. Once you have

cloned Spack, we recommend sourcing the appropriate script for your shell.

This will add Spack to your PATH and enable the use of the `spack` command:

```shell

# For bash/zsh/sh

$ . spack/share/spack/setup-env.sh

# For tcsh/csh

$ source spack/share/spack/setup-env.csh

# For fish

$ . spack/share/spack/setup-env.fish

```

Since `Cargo` is not yet available in the official Spack repositories, you need

to add the Cargo Spack repository to the Spack namespace in your machine. To do

that, download the `spack/` directory in the `Cargo` repository's root to your 

machine (e.g. under `~/projects/cargo/spack`) and execute the following:

```shell

spack repo add ~/projects/cargo/spack/

```

You should now be able to fetch information from the `Cargo` package using

Spack:

```shell

spack info cargo

```

You are now ready to install `Cargo`:

```shell

spack install cargo

```

Include or remove variants with Spack when a custom `Cargo` build is desired.

The available variants are listed below:

| Variant | Command     | Default | Description                        |

|---------|-------------|---------|------------------------------------|

| OFI     | `cargo+ofi` | True    | Use libfabric as transport library |

| UCX     | `cargo+ucx` | False   | Use ucx as transport library       |

> **Attention**

>

> The initial install could take a while as Spack will install build

> dependencies (autoconf, automake, m4, libtool, and pkg-config) as well as

> any dependencies of dependencies (cmake, perl, etc.) if you don’t already

> have these dependencies installed through Spack.

After the installation completes, remember that you first need to load

`Cargo` in order to use it:

```shell

spack load cargo

```

### Building Cargo manually

If you prefer to build and install `Cargo` from sources, you can also do so. 

For the build process to work correctly, the dependencies below will 

need to be available in your system:

| Dependency                                         | Version                   |

|----------------------------------------------------|---------------------------|

| Margo                                              | v0.9.8+                   |

| Argobots                                           | v1.1+                     |

| Mercury                                            | v2.1.0+                   |

| Thallium                                           | v0.10.1+                  |

| libfabric (if `CARGO_TRANSPORT_LIBRARY=libfabric`) | v0.10.1+                  |

| ucx (if `CARGO_TRANSPORT_LIBRARY=ucx`)             | v0.10.1+                  |

| boost program_options                              | v1.71.0+                  |

| boost mpi                                          | v1.71.0+                  |

| boost iostreams (optional, for testing)            | v1.71.0+                  |

| MPI                                                | tested with OpenMPI 4.0.3 |

Once all dependencies are available, you can download build and install

`Cargo` with the following commands:

```shell

## clone the repository

cmake \

  -DCMAKE_PREFIX_PATH:STRING="${PREFIX};${CMAKE_PREFIX_PATH}" \

  -DCMAKE_INSTALL_PREFIX:STRING="${INSTALL_DIR}" \

  -DCARGO_TRANSPORT_LIBRARY:STRING=libfabric \

  -DCARGO_BUILD_TESTS:BOOL=ON \

  ..

make -j8 install

```

These commands will generate and install the Cargo server binary

(`${INSTALL_DIR}/bin/cargo`) as well as the Cargo interface

These commands will generate and install the `Cargo` server binary

(`${INSTALL_DIR}/bin/cargo`) as well as the `Cargo` interface

library (`${INSTALL_DIR}/lib/libcargo.so`) and its headers

(`${INSTALL_DIR}/include/cargo/*`).