README: Remove 'git send-email' mention

The use of 'git send-email' is described in detail in the
contributor guidelines (docs/hacking.html), which are
mentioned in docs/contribute.html, the non-local version
of which is in turn mentioned in README.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

README.md

@@ -64,8 +64,8 @@ Contributing
 
 The libvirt project welcomes contributions in many ways. For most components
 the best way to contribute is to send patches to the primary development
-mailing list, using the `git send-email` command. Further guidance on this
-can be found in the `HACKING` file or on the website:
+mailing list. Further guidance on this can be found in the `HACKING` file
+or on the website:
 
 [https://libvirt.org/contribute.html](https://libvirt.org/contribute.html)