Add instructions for regenerating from XDR files

[leighmcculloch]

What

Add instructions for regenerating from XDR files.

Why

I was regenerating the files and I had to dig around for the instructions for how to do so from other repos.

Known limitations

This PR doesn't completely detail the last step in the process of regenerating the .cs files, which is to reformat the files. It's not clear how reformatting is currently done.

This PR adds an example that works on Linux, and I'm not sure how similar that is to other environments or the environments of the maintainers, so this example might be unhelpful. We could provide multiple examples if that is the case.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[leighmcculloch]

I am also looking into getting the csharp xdrgen language support merged upstream so that the standard xdrgen can be used here and to reduce the complexity of the install and generate process. Ref: https://github.com/stellar/xdrgen/issues/58