Add documentation to README

README.md

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 # java-3rd-order-systems
 A Java class that can be used to solve third-order systems of equations with augmented matrices.
+
+# How to use
+The constructor accepts 12 arguments of type int, found from the coefficients like so:
+
+ax + by + cz = d
+
+ex + fy + gz = h
+
+ix + jy + kz = l
+
+Then, for example, Matrix m = new Matrix(a, b, c, d, e, f, g, h, i, j, k, l);
+
+To solve a matrix, use m.solve(), which returns a double[].
+For example, double[] result = m.solve();
+
+result[0] is the value of x,
+result[1] is the value of y, and
+result[2] is the value of z.