docs: add guidance for populating project-level context (#241)

* docs: add optional project context setup instructions

Add documentation for the optional step of populating project.md with
project details, tech stack, and conventions after running openspec init.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* docs: improve project context section formatting and clarity

- Change heading to "Optional: Populate Project Context" for consistency
- Fix double space typo in "After  openspec init"
- Explicitly reference openspec/project.md file
- Enhance explanation of project.md purpose

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: Claude <[email protected]>

Author: harikrishnan83

README.md

@@ -153,6 +153,17 @@ openspec init
 - If your coding assistant doesn't surface the new slash commands right away, restart it. Slash commands are loaded at startup,
   so a fresh launch ensures they appear
 
+### Optional: Populate Project Context
+
+After `openspec init` completes, you'll receive a suggested prompt to help populate your project context:
+
+```text
+Populate your project context:
+"Please read openspec/project.md and help me fill it out with details about my project, tech stack, and conventions"
+```
+
+Use `openspec/project.md` to define project-level conventions, standards, architectural patterns, and other guidelines that should be followed across all changes.
+
 ### Create Your First Change
 
 Here's a real example showing the complete OpenSpec workflow. This works with any AI tool. Those with native slash commands will recognize the shortcuts automatically.