Add install from source instructions

Update README.md with instructions for downloading and running the
install from source script.

README.md

@@ -89,6 +89,32 @@ sudo /usr/local/share/gcm-core/uninstall.sh
 <a name="linux-install-instructions"></a>
 ### Linux
 
+#### Experimental: install from source helper script
+
+If you would like to help dogfood our new install from source helper script,
+run the following:
+
+1. To ensure `curl` is installed:
+
+```shell
+curl --version
+```
+
+If `curl` is not installed, please use your distribution's package manager
+to install it.
+
+0. To download and run the script:
+
+```shell
+curl -LO https://raw.githubusercontent.com/GitCredentialManager/git-credential-manager/main/src/linux/Packaging.Linux/install-from-source.sh &&
+sh ./install-from-source.sh &&
+git-credential-manager-core configure
+```
+
+__Note:__ You will be prompted to enter your credentials so that the script
+can download GCM's dependencies using your distribution's package
+manager.
+
 #### Ubuntu/Debian distributions
 
 Download the latest [.deb package](https://github.com/GitCredentialManager/git-credential-manager/releases/latest), and run the following: