docs(examples): Add README for gmsh heat equation example

New file: examples/gmsh_heat_equation/README.md (74 lines)

Quick-start documentation covering:
- Problem statement (heat equation with BCs)
- Quick start commands (mesh generation, run example)
- What you get (assembly workflow, matrix extraction)
- Academic usage section directly addressing Issue #183
- Code snippet showing K, M, f extraction for external solvers
- File listing and links to comprehensive tutorial

Provides immediate context for users discovering this example

examples/gmsh_heat_equation/README.md

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+# Heat Equation Example: From Gmsh to Physics
+# Addresses Issue #183: Academic usage without built-in physics
+
+This example demonstrates the complete workflow:
+1. Generate mesh using Gmsh
+2. Load mesh into JuliaFEM
+3. Assemble stiffness matrix and mass matrix
+4. Extract matrices for external solvers (e.g., DifferentialEquations.jl)
+5. Solve the heat equation
+
+## Problem Statement
+
+Solve the transient heat equation on a unit square:
+
+```
+∂u/∂t = α∇²u + f(x,y,t)
+```
+
+with boundary conditions:
+- u = 0 on left edge (Dirichlet)
+- ∂u/∂n = 0 on other edges (Neumann, natural BC)
+
+Initial condition: u(x,y,0) = sin(πx)sin(πy)
+
+## Quick Start
+
+### 1. Generate mesh
+
+```bash
+gmsh -2 unit_square.geo -o unit_square.msh
+```
+
+This creates a triangular mesh of the unit square.
+
+### 2. Run the example
+
+```bash
+julia --project gmsh_heat_equation.jl
+```
+
+## What You Get
+
+The example shows how to:
+- Load Gmsh mesh files
+- Create FEM elements with material properties
+- Assemble global stiffness matrix K and mass matrix M
+- Apply Dirichlet boundary conditions
+- Extract the resulting ODE system: M du/dt = -K u + f
+- Solve using your own time integrator
+
+## For Academic Users (Issue #183)
+
+If you want to use JuliaFEM just for discretization (not the built-in physics):
+
+```julia
+# After assembly, extract the matrices:
+K = problem.assembly.K  # Stiffness matrix (SparseMatrixCSC)
+M = problem.assembly.M  # Mass matrix (SparseMatrixCSC)
+f = problem.assembly.f  # Force vector
+
+# Now use these with DifferentialEquations.jl, Krylov.jl, etc.
+# The ODE system is: M * du/dt = -K * u + f
+```
+
+## Files
+
+- `unit_square.geo` - Gmsh geometry definition
+- `gmsh_heat_equation.jl` - Complete working example
+- `README.md` - This file
+
+## See Also
+
+- Tutorial: `docs/book/gmsh_tutorial.md` (comprehensive step-by-step)
+- Issue #183: https://github.com/JuliaFEM/JuliaFEM.jl/issues/183