switch README from plain text to markdown

no major content updates, just minor wording fixes.
diff --git a/README.txt b/README.md
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rename to README.md
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+++ b/README.md
@@ -1,22 +1,22 @@
-== Building ICD and ICD Test ==
+# Building the OpenCL ICD Loader and Tests
 
-The build system will build ICD Loader library (OpenCL.dll or libOpenCL.so) and
-ICD Loader Test binary (icd_loader_test) and some helper libraries for the test.
+The build system will build ICD Loader library (OpenCL.dll or libOpenCL.so), the
+ICD Loader Test binary (icd_loader_test), and some helper libraries for the test.
 
-=== Linux ===
+## Linux
 
 Run "make"
 
-=== Windows ===
+## Windows
 
 Run "build_using_cmake.bat"
 
-== Running ICD Test ==
+## Running ICD Test
 
-ICD Test can be run using ctest, which is a companion to cmake. It can also be
+The ICD Test can be run using ctest, which is a companion to cmake. It can also be
 run directly by executing icd_loader_test(.exe) executable from the bin folder.
 
-=== Linux ===
+### Linux
 
 1. Add driver stub as an ICD
     echo full/path/to/libOpenCLDriverStub.so > /etc/OpenCL/vendors/test.icd
@@ -24,7 +24,7 @@
 2. Run test using ctest
     make test
 
-=== Windows ===
+### Windows
 
 1. Add driver stub as an ICD by adding appropriate registry value
     Key for 32-bit apps: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Khronos\OpenCL\Vendors
@@ -40,11 +40,10 @@
     cd build
     ctest.exe
 
-== Cleanup ==
+### Cleanup
 
 Manually remove the registry key or .icd files added for running the ICD test.
 
 The "build" and "bin" folders are autogenerated by the build so those may be
 safely deleted without losing any source code (on Linux "make clobber" will
 delete them).
-