add readme

Author: gaowenliang

README.md

@@ -22,8 +22,8 @@ Parameter | YAML element | Symbol | Units
 --- | --- | --- | ---
 Gyroscope "white noise" | `gyr_n` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_g}"> | <img src="https://latex.codecogs.com/svg.latex?{%5Cfrac%7Brad%7D%7Bs%7D%5Cfrac%7B1%7D%7B%5Csqrt%7BHz%7D%7D}">
 Accelerometer "white noise" | `acc_n` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_a}"> | <img src="https://latex.codecogs.com/svg.latex?{%5Cfrac%7Bm%7D%7Bs^2%7D%5Cfrac%7B1%7D%7B%5Csqrt%7BHz%7D%7D}">
-Gyroscope "bias Instability" | `gyr_w` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_b_g}"> | <img src="https://latex.codecogs.com/gif.latex?\frac{rad}{s}&space;\sqrt{Hz}" title="\frac{rad}{s} \sqrt{Hz}" />
-Accelerometer "bias Instability" | `acc_w` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_b_a}"> | <img src="https://latex.codecogs.com/gif.latex?\frac{m}{s^2}&space;\sqrt{Hz}" title="\frac{m}{s^2} \sqrt{Hz}" />
+Gyroscope "bias Instability" | `gyr_w` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_b_g}"> | <img src="http://latex.codecogs.com/svg.latex?\frac{rad}{s}&space;\sqrt{Hz}" title="\frac{rad}{s} \sqrt{Hz}" />
+Accelerometer "bias Instability" | `acc_w` | <img src="https://latex.codecogs.com/svg.latex?{%5Csigma_b_a}"> | <img src="http://latex.codecogs.com/svg.latex?\frac{m}{s^2}&space;\sqrt{Hz}" title="\frac{m}{s^2} \sqrt{Hz}" />
 
 * White noise is at tau=1;