agent: update README.md

Change-Id: I7e46255256ebce1a1e133c53d170539f6c8e1f8c

test/agent/README.md

-# Agent build steps (linux only)
-Last revision: 2021-08-02
+# Agent build steps (GNU/Linux only)
+Last revision: 2021-08-31
 
 # Requirements
 Guile library version 3.0.7 or higher is required. Guile lib may require other 
-dependencies such as libunistring-dev package.
+dependencies in particular libunistring-dev and libgc-deb packages.
 
 Guile can be provided by the distro if available, or built locally. Note that 
 Guile v3.0.7 is quite recent and most likely not yet provided by most linux 
-distros.
+distributions.
 If the required version is available on your distro, just install it using your 
 distro's package manager. Development packages must be installed as well.
 
@@ -29,7 +29,7 @@ stage, Guile must be already available.
 
 ```sh
 cd daemon
-./configure --enable-agent  # you can other options if needed such as --enable-debug
+./configure --enable-agent  # other options can be added if needed such as --enable-debug
 cd test/agent
 make check
 ```