Update README.md

README.md

-# filter_testing
+# COVID-19 Filter Testing
 
+## mask characterization metrics
+- filtration efficiency 
+- fit factor
+
+## conventional test methods
+- Henderson apparatus: generates closed circuit microbial aerosols from a Collison nebulizer at a controlled humidity 
+
+## Resources
+- University of Cambridge [DIY mask efficacy testing](https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic)
+- https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/