Add schema file gen to README

[goneall]

Add the syntax for generating the schema file to the README.md file.

Closes #22

[goneall]

This time I tested the command line. @JPEWdev Feel free to replace this with a better description, just thought it would be helpful to document this special case since it took me some digging to figure it out.

[goneall]

Add -i annotations-file

[bact]

Since shacl2code can be used in non-SPDX context, we may like to have the documentation be more generic, what about:

---

Generating a JSON Schema File

shacl2code can generate a JSON schema using the following command structure:

shacl2code generate -i <input_model> -x <context_url> jsonschema -o <output_schema>

Note that -u or --context-url can be used to reference a local context file as if it were at the specified URL (useful for locally developing before the context is published):

shacl2code generate -i <input_model> -u <context_file> <context_url> jsonschema -o <output_schema>

For instance, to generate the SPDX 3.0.1 JSON schema, you would use:

shacl2code generate \
            -i spdx-model.ttl \
            -i annotations.ttl \
            -u spdx-context.jsonld https://spdx.org/rdf/3.0.1/spdx-context.jsonld \
            jsonschema \
            -o schema.json

Note that the context file (spdx-context.jsonld in this example) should align with the model described in the input files (spdx-model.ttl and annotations.ttl).

For a complete list of available options, run shacl2code generate -h.

---

This will make the doc more generic and at the same time retain the example that is useful for SPDX users.

The above text is based on @goneall text in this PR and from @JPEWdev comment here #22 (comment)