Add exercise assignment for Linear algebra

Author: mlouhivu

Diff for: numpy/linear-algebra/README.md

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+## Linear algebra
+
+1. Construct two symmetric 2x2 matrices **A** and **B**.
+   *Hint: a symmetric matrix can be constructed easily as
+          **Asym** = **A** + **A**^T*
+2. Calculate the matrix product **C** = **A** * **B** using `numpy.dot()`.
+3. Calculate the eigenvalues of matrix **C** with `numpy.linalg.eigvals()`.