EFieldReader: parameter explanations in readme

The reader provides the following models for electric fields:

* For **constant** electric fields it add a constant electric field in the z-direction towards the pixel implants. This is not very physical but might aid in developing and testing new charge propagation algorithms.

* For **linear** electric fields, the field has a constant slope determined by the bias voltage and the depletion voltage. The sensor is depleted either from the implant or the back side, the direction of the electric field depends on the sign of the bias voltage (with negative bias voltage the electric field vector points towards the backplane and vice versa). If the sensor is depleted from the implant side, the electric field is calculated using the formula

* For **linear** electric fields, the field has a constant slope determined by the bias voltage and the depletion voltage. The sensor is depleted either from the implant or the back side, the direction of the electric field depends on the sign of the bias voltage (with negative bias voltage the electric field vector points towards the backplane and vice versa).   The sign of depletion voltage is always ignored.

  If the sensor is depleted from the implant side, the absolute value of the electric field is calculated using the formula

    ```math

    E(z) = \frac{U_{bias} - U_{depl}}{d} + 2 \frac{U_{depl}}{d}\left( 1- \frac{z}{d} \right),

  E(z) = \|frac{U_{bias}| - |U_{depl}|}{d} + 2 \frac{|U_{depl}|}{d}\left( 1- \frac{z}{d} \right),

    ```

    where d is the thickness of the sensor, and $`U_{depl}`$, $`U_{bias}`$ are the depletion and bias voltages, respectively. In case of a depletion from the back side, the electric field is calculated as

    where d is the thickness of the sensor, and $`U_{depl}`$, $`U_{bias}`$ are the depletion and bias voltages, respectively. In case of a depletion from the back side, the absolute value of the electric field is calculated as

    ```math

    E(z) = \frac{U_{bias} - U_{depl}}{d} + 2 \frac{U_{depl}}{d}\left( \frac{z}{d} \right).

  E(z) = \frac{|U_{bias}| - |U_{depl}|}{d} + 2 \frac{|U_{depl}|}{d}\left( \frac{z}{d} \right).

    ```

* For **parabolic** electric fields, a parabola is defined in order to emulate a double-peaked field such as the electric fields observed in sensors after irradiation. The parabola is calculated from the position $`z_{min}`$ and value $`E_{min}`$ of the minimum field in the sensor and the field value at the readout electrode, $`E_{max}`$. The parameters of parabolic equation $`E(z) = az^2 + bz + c`$ then resolve to:

    ```math