Update PROTOC install documentation

We now need to include ABSL in our protobuf
installation guide, so we should update our
documentation.

PiperOrigin-RevId: 672548533

README.md

@@ -18,31 +18,23 @@ Common Lisp.
     Common Lisp code for a given `.proto` file is generated by a plug-in for
     `protoc`, the protocol buffer compiler. The plug-in is written in C++ and
     requires the full version of Google's protocol buffer code to be installed
-    in order to build it, not just the precompiled protoc binaries.
+    in order to build, not just the precompiled protoc binaries. We also
+    require Google's ABSL C++ library to be installed.
 
-    Download and build Google protobuf. Rough instructions are included here for
-    Unix systems. If you have any problems please see the
-    [protobuf installation instructions](https://github.com/protocolbuffers/protobuf/tree/master/src).
-
-    ```shell
-    $ git clone --recursive https://github.com/google/protobuf
-    $ cd protobuf
-    $ ./autogen.sh
-    $ ./configure --prefix=/usr/local
-    $ make
-    $ make check
-    $ sudo make install
-    $ sudo ldconfig
-    ```
+    Depending on your package manager, you may be able to install these libraries
+    through `apt` (or your system's package manager). If you need to isntall
+    from source you can see the example in our continuous integration tests.
 
     Make sure the `protoc` binary is on your `PATH`.
 
 2.  Build the Lisp `protoc` plugin
 
+    We use CMake to install the Lisp protoc plugin.
+
     ```shell
     $ cd cl-protobufs/protoc
-    $ PROTOC_ROOT=/usr/local make     # value of --prefix, above
-    $ sudo cp protoc-gen-cl-pb /usr/local/bin/
+    $ cmake . -DCMAKE_CXX_STANDARD=17
+    $ cmake --build . --target install --parallel 16
     ```
 
     Make sure the installation directory is on your `PATH`.