Upadate the README with instructions for installing gRPC (p4lang#614)

* Upadate the README with instructions for installing  gRPC

Signed-off-by: DScano <[email protected]>

* Decouple the build and install steps

Signed-off-by: DScano <[email protected]>

---------

Signed-off-by: DScano <[email protected]>

Author: Dscano

README.md

@@ -76,9 +76,17 @@ make
 git clone --depth=1 -b v1.43.2 https://github.com/google/grpc.git
 cd grpc/
 git submodule update --init --recursive
+mkdir -p "cmake/build"
+pushd "cmake/build"
+cmake \
+  -DCMAKE_BUILD_TYPE=Release \
+  -DgRPC_INSTALL=ON \
+  -DgRPC_BUILD_TESTS=OFF \
+  -DgRPC_SSL_PROVIDER=package \
+  ../..
 make
 [sudo] make install
-[sudo] ldconfig
+popd
 ```
 - [sysrepo](https://github.com/sysrepo/sysrepo) and all its dependencies: see
   instructions in [proto/README.md](proto/README.md)