Wording tweaks to go with #4810 (#4866)

* mmol/L wording tweak in Readme

* Various wording & clarity tweaks in the Readme

* Heroku template update to reflect mmol allowance tweak

* Update README.md

* Update minimum device requirements

Ran some proper browser test suites, determined actual minimum specs for the main site.

app.json

@@ -93,7 +93,7 @@
       "required": false
     },
     "DISPLAY_UNITS": {
-      "description": "Preferred BG units for the site:'mg/dl' or 'mmol'. (Note that it is *not* 'mmol/L')",
+      "description": "Preferred BG units for the site: 'mg/dl' or 'mmol/L' (or just 'mmol').",
       "value": "mg/dl",
       "required": true
     },