Update install instructions to note issue qitab#402

README.md

@@ -17,12 +17,24 @@ 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.
+    requires Google's protocol buffer compiler as well as the development headers.
 
-    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).
+    **Warning**
+    Due to `protoc`'s dependence on Abseil in v22, the headers required to install
+    the lisp plugin are no longer included. We are actively working on fixing
+    this dependency (#402). Until then, you will need to install a version of
+    protobuf older than v22, but at least v3.15.
+
+    The compiler and development headers can be installed via package manager.
+    ### Ubuntu 22.04
+    ```shell
+    sudo apt install protobuf-compiler libprotobuf-dev libprotoc-dev
+    ```
+
+    ### From source
+    To install from source, rough instructions are included here to install
+    protoc v21 on Unix systems. If you have any problems please see the
+    [protobuf installation instructions](https://github.com/protocolbuffers/protobuf/blob/21.x/src/README.md).
 
     ```shell
     $ git clone --recursive https://github.com/google/protobuf
@@ -39,9 +51,13 @@ Common Lisp.
 
 2.  Build the Lisp `protoc` plugin
 
+    If you installed protoc via package manager, set `PROTOC_ROOT` to `/usr`. If
+    you installed it from source using the instructions above, use the value of
+    `--prefix` that you used.
+
     ```shell
     $ cd cl-protobufs/protoc
-    $ PROTOC_ROOT=/usr/local make     # value of --prefix, above
+    $ PROTOC_ROOT=/usr/local make
     $ sudo cp protoc-gen-cl-pb /usr/local/bin/
     ```