update README.md.

Signed-off-by: 刘明宏 <liuminghong@mail.sdu.edu.cn>

README.md

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 # em3d-MT
 
-#### Introduction
+### 1.Introduction
 em3d-MT is an open-source MATLAB-based package for 3D forward modeling of magnetotelluric (MT) and radiomagnetotelluric (RMT) data. It is built on an edge-based finite element (FE) framework and solves the full-wave Maxwell’s equations, accounting for both conduction and displacement currents. This enables accurate modeling in the RMT frequency range.
 
 The software supports arbitrary anisotropy in both electrical conductivity and dielectric permittivity, allowing flexible simulation of complex geological scenarios. It utilizes unstructured tetrahedral meshes and a customizable geological modeling workflow to accommodate irregular terrain and heterogeneous media.
 
 To address the high computational cost of 3D modeling, em3d-MT integrates a direct linear solver with a double-layer parallelization strategy. This design supports efficient simulation over multiple frequencies using OpenMP-based multithreading and frequency-level parallelization.
 
-#### software framework
+### 2 software framework
 This section provides detailed instructions for installing and configuring the em3d-MT package. The software is developed in MATLAB and requires proper setup of external solvers for efficient computation.
 #### 2.1 System Requirements
 Operating System: Windows or Linux