Cmake readme (#10653)

* quick readme for cmake

* fix wrapping

---------

Co-authored-by: Chris McFarlen <[email protected]>

README.md

@@ -18,6 +18,113 @@ Traffic Server is a high-performance building block for cloud services.
 It's more than just a caching proxy server; it also has support for
 plugins to build large scale web applications.
 
+# Important notice to ATS developers
+
+ATS is transitioning to cmake as its build system.  At the moment, the autotools build is broken and will soon be removed from the repository.  Below is a quick-start guide to cmake:
+
+### Step 1: Configuration
+
+With cmake, you definitely want to create an out-of-source build.  You will give that directory to every cmake command.  For these examples, it will just be `build`
+
+```
+$ cmake -B build
+```
+
+This will configure the project with defaults.
+
+If you want to customize the build, you can pass values for variables on the command line.  Or, you can interactively change them using the `ccmake` program.
+
+```
+$ cmake -B build -DCMAKE_INSTALL_PREFIX=/tmp/ats -DBUILD_EXPERIMENTAL_PLUGINS=ON
+```
+
+-- or --
+
+```
+$ ccmake build
+```
+
+#### Specifying locations of dependencies
+
+To specify the location of a dependency (like --with-*), you generally set a variable with the `ROOT`. The big exception to this is for openssl. This variable is called `OPENSSL_ROOT_DIR`
+
+```
+$ cmake -B build -Djemalloc_ROOT=/opt/jemalloc -DPCRE_ROOT=/opt/edge -DOPENSSL_ROOT_DIR=/opt/boringssl
+```
+
+#### Using presets to configure the build
+
+cmake has a feature for grouping configurations together to make configuration and reproduction easier.  The file CMakePresets.json declares presets that you can use from the command line.  You can provide your own CMakeUserPresets.json and further refine those via inheritance:
+
+```
+$ cmake --preset dev
+```
+
+You can start out your user presets by just copying `CMakePresets.json` and removing everything in `configurePresets`
+
+Here is an example user preset:
+
+```
+
+    {
+      "name": "clang",
+      "hidden": true,
+      "environment": {
+        "LDFLAGS": "-L/opt/homebrew/opt/llvm/lib -L/opt/homebrew/opt/llvm/lib/c++ -Wl,-rpath,/opt/homebrew/opt/llvm/lib/c++ -fuse-ld=/opt/homebrew/opt/llvm/bin/ld64.lld",
+        "CPPFLAGS": "-I/opt/homebrew/opt/llvm/include",
+        "CXXFLAGS": "-stdlib=libc++",
+        "CC": "/opt/homebrew/opt/llvm/bin/clang",
+        "CXX": "/opt/homebrew/opt/llvm/bin/clang++"
+      }
+    },
+    {
+      "name": "mydev",
+      "displayName": "my development",
+      "description": "My Development Presets",
+      "binaryDir": "${sourceDir}/cmake-build-dev-clang",
+      "inherits": ["clang", "dev"],
+      "cacheVariables": {
+        "CMAKE_INSTALL_PREFIX": "/opt/ats-cmake",
+        "jemalloc_ROOT": "/opt/homebrew",
+        "ENABLE_LUAJIT": false,
+        "ENABLE_JEMALLOC": true,
+        "ENABLE_MIMALLOC": false,
+        "ENABLE_MALLOC_ALLOCATOR": true,
+        "BUILD_EXPERIMENTAL_PLUGINS": true,
+        "BUILD_REGRESSION_TESTING": true
+      }
+    },
+```
+
+And then use it like:
+
+```
+cmake --preset mydev
+```
+
+## Building the project
+
+```
+$ cmake --build build
+```
+
+```
+$ cmake --build build -t traffic_server
+```
+
+## running tests
+
+```
+$ cd build
+$ ctest
+```
+
+## installing
+
+```
+$ cmake --install build
+```
+
 ## DIRECTORY STRUCTURE
 ```
 trafficserver ............. Top src dir