Update README

Add comparison explanation and clarify error propagation description.

Author: hlouzada

README.md

@@ -6,12 +6,28 @@
 
 Small library for uncertainty calculations and error propagation.
 
-The uncertainty calculations are in accordance with gaussian’s propagation, as calculated by an analytical method:
+### Error propagation:
+
+The uncertainty is calculated analytically in accordance with the gaussian propagation:
 
 <p align="center">
 <img src="https://latex.codecogs.com/svg.latex?%5Cdpi%7B120%7D%20%5CDelta_f%20%3D%20%5Csqrt%7B%5Cleft%28%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20x%7D%5Cright%29%5E2%7B%5CDelta_x%7D%5E2%20&plus;%20%5Cleft%28%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20y%7D%5Cright%29%5E2%7B%5CDelta_y%7D%5E2%20&plus;%20%5Cleft%28%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20z%7D%5Cright%29%5E2%7B%5CDelta_z%7D%5E2%20&plus;%20...%7D">
 </p>
 
+
+### To compare two labfloats it is used the following methods:
+
+Assuming:  
+<img src="https://latex.codecogs.com/svg.image?%5Cbg_white%20%5Cinline%20labfloat(x_i,%5Csigma_i)%5Cequiv%20x_i%20%5Cpm%20%5Csigma_i">
+
+- If they are equal they must satisfy:  
+<img src="https://latex.codecogs.com/svg.image?%5Cbg_white%20%5Cinline%20%5Cleft%7Cx_1%20-%20x_2%5Cright%7C%20%3C%202(%5Csigma_1%20&plus;%20%5Csigma_2)">
+ 
+- If they are different they must satisfy:  
+<img src="https://latex.codecogs.com/svg.image?%5Cbg_white%20%5Cinline%20%5Cleft%7Cx_1%20-%20x_2%5Cright%7C%20%3E%203(%5Csigma_1%20&plus;%20%5Csigma_2)">
+
+> **_NOTE:_**  Two labfloats can be not different and not equal at the same time by these methods.
+
 Made by and for Physics Laboratory students in IFSC, who can't use uncertainties.py because of mean’s absolute deviation used in its calculation.
 
 ## Usage
@@ -57,3 +73,5 @@ pip install git+https://github.com/phisgroup/labfis.py@development
  11. ["Strategies for Variance Estimation"](http://www.sagepub.com/upm-data/6427_Chapter_4__Lee_%28Analyzing%29_I_PDF_6.pdf)  (PDF). p. 37. Retrieved  2013-01-18.
  12. Harris, Daniel C. (2003), [_Quantitative chemical analysis_](https://books.google.com/books?id=csTsQr-v0d0C&pg=PA56)(6th ed.), Macmillan, p. 56, [ISBN](https://en.wikipedia.org/wiki/ISBN_(identifier) "ISBN (identifier)")  [978-0-7167-4464-1](https://en.wikipedia.org/wiki/Special:BookSources/978-0-7167-4464-1 "Special:BookSources/978-0-7167-4464-1")
  13. ["Error Propagation tutorial"](http://www.foothill.edu/psme/daley/tutorials_files/10.%20Error%20Propagation.pdf)  (PDF).  _Foothill College_. October 9, 2009. Retrieved  2012-03-01.
+ 14. Helene,  O.;  Vanin,  V.. Tratamento  estatístico  de  dados  em  física experimental. São Paulo: Editora Edgard Blücher, 1981.
+ 15. Vuolo,  J.  E.. Fundamentos  da  teoria  de  erros.  2.  ed.  São  Paulo:  Editora Edgard Blücher, 1993.