Document the KEEP_ARCHIVE env variable

Author: swiatekm

README.md

@@ -108,6 +108,11 @@ To build a local version of the agent for development, run the command below. Th
 DEV=true EXTERNAL=true SNAPSHOT=true PLATFORMS=linux/amd64 PACKAGES=tar.gz mage -v package
 ```
 
+If you build the same agent package often (when running integration tests, for example),
+you can also set KEEP_ARCHIVE=true in your environment. The packaging step will then
+avoid deleting the binary archive after the package is generated, removing the need to
+re-download binaries on every invocaion.
+
 The resulting package will be produced in the build/distributions directory. The version is controlled by the value in [version.go](version/version.go).
 To install the agent extract the package and run the install command: