Add README explaining NoSlop is a collection of prompt files

[Copilot]

The existing README was two lines with no context on what the project actually is or how to use it.

Changes

• What is this — states explicitly that the repo is plain-text prompt files only; no code, no dependencies
• What is "slop" — defines the term so the project goal is immediately clear to new visitors
• How to use — four-step guide: browse → copy → paste into system prompt → profit
• Contributing — directs contributors to add .md/.txt prompt files via PR

Original prompt

for now have a readme.md explaining how its js a bunch of prompt files