Add content to README (KhronosGroup#1427)

Fill in the placeholder readme with some basic information on building and
running the project. Information on the conformance submission process and
contributing are also included.

Should help close a few issues referenced in
KhronosGroup#1096

I don't think this is all the information we want, but is a starting point
from which we can progress. For example, adding the android build instructions
from KhronosGroup#1021

README.md

@@ -1,2 +1,115 @@
-# OpenCL-CTS
-The OpenCL Conformance Tests
+# OpenCL Conformance Test Suite (CTS)
+
+This it the OpenCL CTS for all versions of the Khronos
+[OpenCL](https://www.khronos.org/opencl/) standard.
+
+## Building the CTS
+
+The CTS supports Linux, Windows, macOS, and Android platforms. In particular,
+GitHub Actions CI builds against Ubuntu 20.04, Windows-latest, and
+macos-latest.
+
+Compiling the CTS requires the following CMake configuration options to be set:
+
+* `CL_INCLUDE_DIR` Points to the unified
+  [OpenCL-Headers](https://github.com/KhronosGroup/OpenCL-Headers).
+* `CL_LIB_DIR` Directory containing the OpenCL library to build against.
+* `OPENCL_LIBRARIES` Name of the OpenCL library to link.
+
+It is advised that the [OpenCL ICD-Loader](https://github.com/KhronosGroup/OpenCL-ICD-Loader)
+is used as the OpenCL library to build against. Where `CL_LIB_DIR` points to a
+build of the ICD loader and `OPENCL_LIBRARIES` is "OpenCL".
+
+### Example Build
+
+Steps on a Linux platform to clone dependencies from GitHub sources, configure
+a build, and compile.
+
+```sh
+git clone https://github.com/KhronosGroup/OpenCL-CTS.git
+git clone https://github.com/KhronosGroup/OpenCL-Headers.git
+git clone https://github.com/KhronosGroup/OpenCL-ICD-Loader.git
+
+mkdir OpenCL-ICD-Loader/build
+cmake -S OpenCL-ICD-Loader -B OpenCL-ICD-Loader/build \
+      -DOPENCL_ICD_LOADER_HEADERS_DIR=$PWD/OpenCL-Headers
+cmake --build ./OpenCL-ICD-Loader/build --config Release
+
+mkdir OpenCL-CTS/build
+cmake -S OpenCL-CTS -B OpenCL-CTS/build \
+      -DCL_INCLUDE_DIR=$PWD/OpenCL-Headers \
+      -DCL_LIB_DIR=$PWD/OpenCL-ICD-Loader/build \
+      -DOPENCL_LIBRARIES=OpenCL
+cmake --build OpenCL-CTS/build --config Release
+```
+
+## Running the CTS
+
+A build of the CTS contains multiple executables representing the directories in
+the `test_conformance` folder. Each of these executables contains sub-tests, and
+possibly smaller granularities of testing within the sub-tests.
+
+See the `--help` output on each executable for the list of sub-tests available,
+as well as other options for configuring execution.
+
+If the OpenCL library built against is the ICD Loader, and the vendor library to
+be tested is not registered in the
+[default ICD Loader location](https://github.com/KhronosGroup/OpenCL-ICD-Loader#registering-icds)
+then the [OCL_ICD_FILENAMES](https://github.com/KhronosGroup/OpenCL-ICD-Loader#table-of-debug-environment-variables)
+environment variable will need to be set for the ICD Loader to detect the OpenCL
+library to use at runtime. For example, to run the basic tests on a Linux
+platform:
+
+```sh
+OCL_ICD_FILENAMES=/path/to/vendor_lib.so ./test_basic
+```
+
+### Offline Compilation
+
+Testing OpenCL drivers which do not have a runtime compiler can be done by using
+additional command line arguments provided by the test harness for tests which
+require compilation, these are:
+
+* `--compilation-mode` Selects if OpenCL-C source code should be compiled using
+  an external tool before being passed on to the OpenCL driver in that form for
+  testing. Online is the default mode, but also accepts the values `spir-v`, and
+  `binary`.
+
+* `--compilation-cache-mode` Controls how the compiled OpenCL-C source code
+  should be cached on disk.
+
+* `--compilation-cache-path` Accepts a path to a directory where the compiled
+  binary cache should be stored on disk.
+
+* `--compilation-program` Accepts a path to an executable (default:
+   cl_offline_compiler) invoked by the test harness to perform offline
+   compilation of OpenCL-C source code.  This executable must match the
+   [interface description](test_common/harness/cl_offline_compiler-interface.txt).
+
+## Generating a Conformance Report
+
+The Khronos [Conformance Process Document](https://members.khronos.org/document/dl/911)
+details the steps required for a conformance submissions.
+In this repository [opencl_conformance_tests_full.csv](test_conformance/submission_details_template.txt)
+defines the full list of tests which must be run for conformance. The output log
+of which must be included alongside a filled in
+[submission details template](test_conformance/submission_details_template.txt).
+
+Utility script [run_conformance.py](test_conformance/run_conformance.py) can be
+used to help generating the submission log, although it is not required.
+
+Git [tags](https://github.com/KhronosGroup/OpenCL-CTS/tags) are used to define
+the version of the repository conformance submissions are made against.
+
+## Contributing
+
+Contributions are welcome to the project from Khronos members and non-members
+alike via GitHub Pull Requests (PR). Alternatively, if you've found a bug or have
+a questions please file an issue in the GitHub project. First time contributors
+will be required to sign the Khronos Contributor License Agreement (CLA) before
+their PR can be merged.
+
+PRs to the repository are required to be `clang-format` clean to pass CI.
+Developers can either use the `git-clang-format` tool locally to verify this
+before contributing, or update their PR based on the diff provided by a failing
+CI job.