Update Build Instructions and Example Configuration File

[jscottbranson]

High Level Overview of Change

Revisions to the BUILD.md file for more congruent language "rippled" -> "xahaud". Also, include default [ips] section in the example config file.

It might be ideal for people/projects to rely on the configuration files in this repo, rather than making their own default config files (i.e., use cfg/xahau-example.cfg in the xahaud repo when installing xahaud).

This PR also rewrites much of the contents of "installing", as they were not relevant to xahaud.

Type of Change

• 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 not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release