Update README.md to clarify deprecated methods.

[nujust]

Hello.

It's difficult to know which method is recommended by reading API in README.md.
How about changing to following description?

### API

* **arduinoFFT**(double *vReal, double *vImag, uint16_t samples, double samplingFrequency);
Constructor
* **~arduinoFFT**(void);
Destructor
* **ComplexToMagnitude**();
* **Compute**(uint8_t dir);
Calcuates the Fast Fourier Transform.
* **DCRemoval**();
Removes the DC component from the sample data.
* **MajorPeak**();
Looks for and returns the frequency of the biggest spike in the analyzed signal.
* **Revision**(void);
Returns the library revision.
* **Windowing**(uint8_t windowType, uint8_t dir);
Performs a windowing function on the values array. The possible windowing options are:
    * FFT_WIN_TYP_RECTANGLE
    * FFT_WIN_TYP_HAMMING
    * FFT_WIN_TYP_HANN
    * FFT_WIN_TYP_TRIANGLE
    * FFT_WIN_TYP_NUTTALL
    * FFT_WIN_TYP_BLACKMAN
    * FFT_WIN_TYP_BLACKMAN_NUTTALL
    * FFT_WIN_TYP_BLACKMAN_HARRIS
    * FFT_WIN_TYP_FLT_TOP
    * FFT_WIN_TYP_WELCH

Following methods are deprecated and may be removed on future revisions.
* **arduinoFFT**(void);
* **ComplexToMagnitude**(double *vReal, double *vImag, uint16_t samples);
* **Compute**(double *vReal, double *vImag, uint16_t samples, uint8_t dir);
* **Compute**(double *vReal, double *vImag, uint16_t samples, uint8_t power, uint8_t dir);
* **DCRemoval**(double *vData, uint16_t samples);
* **MajorPeak**(double *vD, uint16_t samples, double samplingFrequency);
* **Windowing**(double *vData, uint16_t samples, uint8_t windowType, uint8_t dir);
* **Exponent**(uint16_t value);