update README

README.md

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+
+This package aims to provide a simple way to combine point group symmetries (e.g., C4v, C3v) with internal symmetries (e.g., SU(2), U(1)) within the framework of [TensorKit.jl](https://github.com/Jutho/TensorKit.jl). 
+
+A minimal example of using this package is as follows. 
+Suppose we aim to generate a SU(2)-symmetric PEPS on the square lattice while ensuring that the local tensor remains invariant under reflections and rotations.
+The internal SU(2) symmetry will be handled by TensorKit.jl, while the spatial symmetries can further be handled by this package:
+
+```julia 
+using TensorKit
+using SpatiallySymmetricTensor 
+```
+
+We first specify the physical/virtual space for the PEPS tensor
+```julia
+# specify the virtual space of the PEPS tensor
+V = SU2Space(1//2=>1, 0=>1) 
+# specify the physical space of the PEPS tensor
+P = SU2Space(1//2=>1) 
+# generate a zero symmetric tensor
+T0 = zeros(ComplexF64, P, V^4)  
+```
+
+Compared to a plain tensor, the SU(2)-symmetric tensor contains fewer free parameters.
+Imposing spatial symmetries will further reduce the number of free parameters in the symmetric tensor.
+In our case, we want to look for the symmetric tensors that belong to the A1 representation of the point group C4v. 
+```julia
+sols = find_solutions(C4v(), T0, :A1)
+```
+The returned `sols` is a vector of linear independent symmetric tensors which (i) have the same internal symmetric structure as `T0` and (ii) belong to the A1 representation of C4v. 
+`sols` will contain only two entries, implying that we are left with only one free parameter after imposing the spatial symmetry. 
+In the end, we can parametrize the PEPS tensor as, for example, 
+```julia
+function PEPS_tensor(x) 
+    coefficients = [1.0, x]
+    return sum(coefficients .* sol)
+end
+```
+where `x` is the free parameter.