docs(io): add I/O module documentation for mesh import/export

New 120-line documentation file describing:
- Supported mesh formats (Abaqus .inp, Code Aster .med/.rmed, Gmsh .msh)
- File organization (core readers vs legacy vendor package infrastructure)
- Design philosophy (weak dependencies, two-tier strategy)
- Future work (export formats, import enhancements, consolidation)
- Usage examples and dependency requirements

Documents the mesh I/O architecture and reader strategy.

src/io/README.md

@@ -0,0 +1,120 @@
+# I/O Module - Mesh Import/Export
+
+**Purpose:** Read and write mesh files from various FEM software packages.
+
+## Supported Formats
+
+### Abaqus (.inp)
+**Status:** ✅ Active  
+**Files:**
+- `AbaqusReader.jl` - Main reader with keyword parsing
+- `abaqus_reader.jl` - Simplified reader
+- `abaqus_download.jl` - Download example meshes
+- `keyword_register.jl` - Keyword parsing system
+
+**Use:** `mesh = read_abaqus("model.inp")`
+
+### Code Aster (.med, .rmed)
+**Status:** ⚠️ Requires HDF5 (weak dependency)  
+**Files:**
+- `AsterReader.jl` - Main Aster reader
+- `aster_reader.jl` - Simplified reader
+- `read_aster_mesh.jl` - HDF5-based mesh reading (NOT included - needs HDF5.jl)
+- `read_aster_results.jl` - HDF5-based results reading (NOT included)
+
+**Strategy:** Implement as weak dependency - only load if HDF5.jl available
+
+### Gmsh (.msh)
+**Status:** ✅ Active  
+**Files:**
+- `gmsh_reader.jl` - Gmsh mesh file parser
+
+**Use:** `mesh = read_gmsh("mesh.msh")`
+
+## File Organization
+
+### Core Readers
+- `io.jl` - Common I/O utilities
+- `gmsh_reader.jl` - Gmsh format
+- `abaqus_reader.jl` - Simple Abaqus reader
+- `aster_reader.jl` - Simple Aster reader (no HDF5)
+
+### Legacy Vendor Package Infrastructure
+- `AbaqusReader.jl` - Full Abaqus reader (from vendor package)
+- `AsterReader.jl` - Full Aster reader (from vendor package)
+- `keyword_register.jl` - Keyword parsing system
+- `parse_mesh.jl` - Generic mesh parsing
+- `parse_model.jl` - Model structure parsing
+- `create_surface_elements.jl` - Surface element extraction
+
+### Disabled (Require HDF5)
+- `read_aster_mesh.jl` - ❌ Commented out (needs HDF5.jl)
+- `read_aster_results.jl` - ❌ Commented out (needs HDF5.jl)
+
+## Design Philosophy
+
+### Weak Dependencies
+For optional formats requiring heavy dependencies (HDF5):
+```julia
+# In Project.toml
+[extras]
+HDF5 = "..."
+
+# In src/io/
+if isdefined(Main, :HDF5)
+    include("read_aster_mesh.jl")
+end
+```
+
+### Two-Tier Strategy
+1. **Simple readers** - Basic functionality, minimal dependencies
+2. **Full readers** - Complete keyword support, complex features
+
+Users can choose based on needs.
+
+## Future Work
+
+### Export Formats
+- VTK/VTU for visualization
+- Exodus II for multi-physics
+- JSON for web applications
+
+### Import Enhancements
+- NASTRAN (.bdf, .nas)
+- ANSYS (.cdb)
+- CalculiX (.inp)
+
+### Consolidation
+- Unify simple vs full reader approaches
+- Document which reader to use when
+- Benchmarks for large meshes
+
+## Usage Examples
+
+```julia
+using JuliaFEM
+
+# Abaqus
+mesh = read_abaqus("cantilever.inp")
+
+# Gmsh (if gmsh_jll available)
+mesh = read_gmsh("geometry.msh")
+
+# Aster (if HDF5 available)
+mesh = read_aster("model.med")
+```
+
+## Dependencies
+
+**Required:**
+- None (pure Julia)
+
+**Optional:**
+- `gmsh_jll` - For Gmsh API access
+- `HDF5.jl` - For Code Aster .med files
+- `Downloads.jl` - For downloading example meshes
+
+---
+
+**Maintainer:** JuliaFEM Team  
+**Last Updated:** November 21, 2025