ReadMe: Convert to Markdown, various cleanup.

* Remove references to outdated stuff (OsDrawer, old OpenGrok)
 * Removed GitHub as a code search option (not nearly as flexible
   as OpenGrok, and updated bi-monthly or so instead of daily.)
 * General reorganization.

Fixes #11842.

ReadMe.md

@@ -0,0 +1,57 @@
+Haiku
+=======================
+[Homepage](https://www.haiku-os.org/)
+| [Mailing Lists](https://www.haiku-os.org/community/ml)
+| [IRC Channels](https://www.haiku-os.org/community/irc)
+| [Issue Tracker](https://dev.haiku-os.org/)
+| [API docs](https://api.haiku-os.org)
+
+Haiku is an open-source operating system that specifically targets personal
+computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn
+and yet very powerful.
+
+Goals
+------------
+ * Sensible defaults with minimal configuration required.
+ * Clean, clear, concise code.
+ * Unified desktop environment.
+
+Trying Haiku
+---------------
+Haiku provides pre-built [nightly images](http://download.haiku-os.org/) and
+[release images](https://www.haiku-os.org/get-haiku). Haiku is compatible
+with a large variety of hardware, but in case you don't want to "take the
+plunge" and install Haiku on bare metal, you can install it on a virtual
+machine (VM) instead. If you've never used a VM before, you can follow one of
+the ["Emulating Haiku"](https://www.haiku-os.org/guides/virtualizing/) guides.
+
+Compiling Haiku
+---------------
+See `ReadMe.Compiling`.
+
+Contributing
+-------------------
+Haiku is a meritocratic open source project with a large variety of tasks. Even
+if you can't write code, *you can still help*! Haiku needs designers,
+(technical) writers, translators, testers... Get involved and help out!
+
+### Contributing code
+If you're submitting a patch to us, please make sure you're following the
+[patch submitting guidelines](https://dev.haiku-os.org/wiki/CodingGuidelines/SubmittingPatches).
+
+If you're having trouble finding something in the source tree, you can use
+one of our OpenGrok servers:
+
+ * http://grok.bikemonkey.org/source (provided by Landon Fuller)
+ * http://code.metager.de/source/xref/haiku (provided by MetaGer)
+
+### Contributing documentation
+The main piece of documentation that still needs work are the API docs (found
+in the tree at `src/docs/user`). Just find an undocumented class, write
+documentation for it, and [submit a patch](https://dev.haiku-os.org/wiki/CodingGuidelines/SubmittingPatches).
+
+### Contributing translations
+See [wiki:i18n](https://dev.haiku-os.org/wiki/i18n).
+
+### Contributing software ports
+See [HaikuPorts](https://bitbucket.org/haikuports/haikuports/).