Rename README.markdown to README.md.

README.md

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+What is Grumble?
+================
+
+Grumble is an implementation of a server for the Mumble voice chat system. It is an alternative to Murmur, the typical Mumble server.
+
+Compiling Grumble from source
+=============================
+
+You must have a Go 1 environment installed to build Grumble. Those are available at:
+
+https://golang.org/dl/
+
+Once Go is installed, you should set up a GOPATH to avoid clobbering your Go environment's root directory with third party packages.
+
+Set up a GOPATH. On Unix, do something like this
+
+    $ export GOPATH=$HOME/gocode
+    $ mkdir -p $GOPATH
+
+and on Windows, do something like this (for cmd.exe):
+
+    c:\> set GOPATH=%USERPROFILE\gocode
+    c:\> mkdir %GOPATH%
+
+Then, it's time to install Grumble. The following line should do the trick:
+
+    $ go get mumble.info/grumble/cmd/grumble
+
+And that should be it. Grumble has been built, and is available in $GOPATH/bin as 'grumble'.