Document files created by bd init and clarify .gitattributes should be committed

Fixes #299

Added comprehensive documentation to help new users understand what files
should be committed vs ignored after running bd init:

1. README.md:
   - New "Files Created by bd init" section
   - Clear lists of files to commit vs ignore
   - Explains purpose of .gitattributes for team collaboration

2. docs/GIT_INTEGRATION.md:
   - Enhanced .gitattributes section with IMPORTANT notice
   - Explains why it must be committed (merge driver config)
   - Lists benefits of intelligent JSONL merging

Key clarification: .gitattributes should be COMMITTED (not ignored) as it
configures git merge behavior for the entire team.

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

Co-Authored-By: Claude <noreply@anthropic.com>

docs/GIT_INTEGRATION.md

@@ -434,6 +434,8 @@ export BEADS_NO_DAEMON=1  # Direct mode
 
 ### Recommended .gitattributes
 
+**IMPORTANT:** The `.gitattributes` file should be **committed to git**, not ignored. It configures merge behavior for the entire team.
+
 ```
 # Intelligent merge driver for JSONL (auto-configured by bd init)
 .beads/beads.jsonl merge=beads
@@ -442,6 +444,11 @@ export BEADS_NO_DAEMON=1  # Direct mode
 .beads/*.jsonl text diff
 ```
 
+This file is automatically created by `bd init` and is essential for:
+- Preventing spurious merge conflicts in `.beads/beads.jsonl`
+- Enabling field-level 3-way merging instead of line-by-line
+- Ensuring all team members get intelligent JSONL merging
+
 ### Git LFS Considerations
 
 **Do NOT use Git LFS for `.beads/beads.jsonl`:**