feat(docs): add WHCM support documentation

docs/on-windows-hcm-support.md

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+**Carbon Component supports High Contrast mode for Windows 10 on Chrome, Edge,
+and Internet Explorer.** Notable by it's absence is Firefox. For background on
+why you need to understand Windows High Contrast Mode and how the three browser
+manufacturers choose to handle it.
+
+### What is Windows High Contrast Mode
+
+Windows High Contrast Mode (WHCM) is an accessibility feature in Windows 10 that
+enables users to select one of four built in themes (two dark and two light) or
+specify their own custom colors to create a theme of their own. This makes it
+impossible to predict how your content will appear to a user with WHCM enabled.
+To mitigate this Microsoft have a proprietary CSS extension which when used as a
+media query allows you to detect WHCM and set a CSS system color as a fallback
+should your visual design lose some amount of usability with this accessibility
+affordance enabled.
+
+```
+@media screen and (-ms-high-contrast: active) {
+  /* All high contrast styling rules */
+
+}
+@media screen and (-ms-high-contrast: black-on-white) {
+  div { background-image: url('image-bw.png');  }
+
+}
+@media screen and (-ms-high-contrast: white-on-black) {
+  div { background-image: url('image-wb.png');  }
+
+}
+```
+
+[From the MDN on -ms-high-contrast](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/-ms-high-contrast#Example)
+
+### Edge/Internet Explorer
+
+Microsoft's browsers Edge and Internet Explorer obviously have ideal support for
+WHCM applying a users chosen theme wholesale to the web content and allowing
+developers to target problematic CSS generated graphics or SVGs via their
+extension and set safe fallbacks using CSS system colors.
+
+### Chrome
+
+Chrome does not support WHCM instead prompting the user to install a
+[Google created High Contrast extension](https://chrome.google.com/webstore/detail/high-contrast/djcfdncoelnlbldjfhinnjlhdjlikmph?hl=en)
+into their browser. This extension has no interoperability with WHCM or a user's
+custom themes; instead users must select from one of six pre-programmed color
+schemes. The schemes filter a site's content and are generally much more
+forgiving on graphics heavy designs.
+
+### Firefox
+
+Firefox has full support and interoperability with WHCM allowing web content to
+be displayed in a user's custom theme. They do not however support Microsoft's
+CSS extension. This makes reliably targetting inaccessible or unusable graphics
+and setting safe fallbacks very difficult and error prone due to the infinte
+customizability of WHCM.
+[There is an open proposal](https://bugzilla.mozilla.org/show_bug.cgi?id=1506364)
+for a `prefers-contrast` media query which we will be monitoring.
+
+### Carbon support for WHCM moving forward
+
+After raising the issue and conferring with our colleagues in IBM Accessibility,
+we've made the decision to support WHCM in Edge, Internet Explorer, and Chrome
+moving forward. We want Carbon components to be accessible to all our users. And
+we can be confident that WHCM users will have a better overall experience on one
+of those browsers.