kpatch-build and README.md update

* `kpatch-build`
Sometimes the downloading of the kernel source tarball via `us.archive.ubuntu.com` can be very slow (20KB/s), switching to a close Ubuntu archive mirror (in my case `au.archive.ubuntu.com` was 25 times faster) will greatly speed up the download.

Generally it is a better idea to use the main archive instead of US specific archive (for load balancing and failover purpose), that's why I changed the URL. 

See the host output below.

```bash
$ host us.archive.ubuntu.com
us.archive.ubuntu.com has address 91.189.91.13
us.archive.ubuntu.com has address 91.189.91.14
us.archive.ubuntu.com has address 91.189.91.15
us.archive.ubuntu.com has IPv6 address 2001:67c:1562::13
us.archive.ubuntu.com has IPv6 address 2001:67c:1562::14
us.archive.ubuntu.com has IPv6 address 2001:67c:1562::15

$ host archive.ubuntu.com
archive.ubuntu.com has address 91.189.92.200
archive.ubuntu.com has address 91.189.92.201
archive.ubuntu.com has address 91.189.88.153
archive.ubuntu.com has address 91.189.91.13
archive.ubuntu.com has address 91.189.91.14
archive.ubuntu.com has address 91.189.91.15
archive.ubuntu.com has IPv6 address 2001:67c:1360:8c01::19
archive.ubuntu.com has IPv6 address 2001:67c:1360:8c01::18

$ host au.archive.ubuntu.com
au.archive.ubuntu.com is an alias for mirror.aarnet.edu.au.
mirror.aarnet.edu.au has address 202.158.214.106
mirror.aarnet.edu.au has IPv6 address 2001:388:30bc:cafe::beef
```

* `README.md`
Broke up long lines, tried to use GitHub flavored markdown to beautify the `README.md`.

Right now the `REAME.md` has mixed markdown syntaxes, may need an overhaul soon ;-D
Agree. So it should be good to just change the URL from `http://us.archive.ubuntu.com/ubuntu/pool/main/l/linux` to `http://archive.ubuntu.com/ubuntu/pool/main/l/linux` which does make sense ;-D