Add comprehensive build instructions and usage documentation

[Copilot]

This PR addresses the confusion around building and using the chartify tool by adding comprehensive build targets and documentation that were missing from the repository.

Changes Made

Makefile Enhancements

Added essential build targets that were requested:

• make build - builds both chartify and chartreposerver binaries
• make clean - removes built binaries
• make install - installs binaries to $GOPATH/bin
• make all - default target that builds everything
• Individual targets build-chartify and build-chartreposerver for granular control

README Documentation

Significantly expanded the README with:

• Prerequisites section - specifies Go 1.24.0+ and Helm 3.x requirements
• Building and Installation section - clear instructions for both local builds and Go install
• Enhanced CLI documentation - detailed examples and usage for both tools:
  • chartify examples with various flags and use cases
  • chartreposerver usage instructions
  • Complete help output documentation
• Development section - information about testing and linting

Repository Maintenance

• Added built binaries (chartify, chartreposerver) to .gitignore to prevent accidental commits
• All changes are backwards compatible and don't affect existing functionality

Before/After Comparison

Before: Users were confused about how to build and run the tool, with no build section in Makefile and minimal usage instructions.

After: Users can now easily:

# Build the project
make build

# Get help
./chartify -h
./chartreposerver

# Clean up
make clean

# Install system-wide
make install

The documentation now provides clear examples for common use cases like converting charts with dependencies, including CRDs, and applying patches.

Fixes #65.

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