Updated missing relative units in css.md

Few relative units were missing from the cheat sheet which have been updated

Author: ofsaksham

css.md

@@ -122,6 +122,8 @@ Units are used for interpreting length in your css code value. These are used in
     | vh | relative to the viewport's height, for eg.: ```6vh``` would be ```6% of the height``` of the viewport  |
     | vmax |  relative to the viewport's bigger dimension which could be either the height or the width, for eg.: ```3vmax``` would mean ```3vh``` if the height is more than the width and would mean ```3vw``` otherwise | 
     | vmin | similar to  vmax, differs because it would consider the smaller dimension of either the height or the width  | 
+    | ch | it is relative to the width of zero (0) |
+    | ex | it is relative to the x height of the current font |
 
 > relative units are much more preferred nowadays as their are just too many devices with varying screen heights, widths, pixel densities etc.