Added 'noslidenumbers' to "Package Options"

I added the 'noslidenumbers' option to the "Package Options" section with a
description of what it does.

Additionally, at the top of the "Package Options" I put how to call theme
options. When first working with mtheme, I struggled to find where to put the
package options, so I figured new users would find it helpful to have in the
ReadMe. It may be unnecessary since it's like every other Beamer theme in that
regard. Still, since it's a non-standard theme, I think it's good for a new user
to be told explicitly.

README.md

@@ -40,6 +40,9 @@ can run the following command
 
 #### Package options
 
+To use any of options below, call them when invoking *mtheme* in the header of
+your document: `\usetheme[<options>]{m}`
+
 The `usetitleprogressbar` option adds a thin progress bar similar to the section
 progress bar underneath *each* frame title
 
@@ -65,6 +68,9 @@ Option `usetotalslideindicator` creates slide numbering in lower right corner
 in following format: #current/#total. By default, just current page number is
 printed.
 
+Option `noslidenumbers` omits slide numbers entirely.
+
+
 #### Title formatting
 
 The main title, section titles, and frame titles are all formatted according
@@ -82,6 +88,7 @@ Note that `\MakeLowercase` and `\MakeUppercase` can have unexpected behaviour
 in math mode, are disabled when `protectframetitle` is used, and cause crashes
 when an unprotected frametitle appears on a slide with `allowframebreaks`.
 
+
 #### Commands
 
 The `\plain{title=[]}{body}` command sets a slide in plain dark colors