Use signing subkey

The signature was made using `0xBECFA3C1AE191D15`, and has to be used with the signing key, not the root key.

I can verify this with my own key -- using the keyid doesn't work:

```
 ~   echo "$(uname -a)" | gpg --armor --clearsign --default-key 0xB1A9D5A2A605F794
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

-----BEGIN PGP SIGNATURE-----
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=TVTZ
-----END PGP SIGNATURE-----
 ~  
 ~  gpg
gpg: Go ahead and type your message ...
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

-----BEGIN PGP SIGNATURE-----

iQIcBAEBCgAGBQJX5aWdAAoJECaAG7YBLqXZi1EP/3R4oOdkXqZXcskLinux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/LinuxwzfjBXa68
oZeKxTB8i74tSPXp0SL26+ULOQ6GRJdIFod2MQtqfjeu6dyNEbIBF1pHWnyLx4Bn
p/+ROoOTiBSFEWPts++yYhmo0tS0cXPv6QPCYqj4mPkJe6u8wVp5hkoyujL/k9bs
cAZSbeyV/hggS0rFTN4/5AeUky4LJPrWYkAiln7D0PVQeZc6DFlDpeup1Az7hWV5
ImRglAfoacNq+0LWslnc51/4knFGC/k4RS/QAyfUNJG
/yy/ZZs6FNc7FjyZkw87E
yRqqSPkuL64BmzNxmfKnwgMAesaq8D674lRb7b9TC8sQuuelcbgPkCCDioRmCSWh
+NIe+pwWLIXHSwQntO2FblGFL+IeDYBZy3P5nO+N12EHn2oS2psep04STq5cjRaa
PTMopcDsThzXljn8b6p+Iu2BaFiMkEwpAD8f0knR4DZzorpgMjIV0mEdeDuTzC1L
dPHc7uZsTSSTEgxm7JO8x1h3hfwqX+K
vVhmo0SgvwexqsmH7+b6j948RPGSCGBys
wS8HEQgzgznQYSxqnCHvuDT9cIuyuCi9BZfqvRy3NSa+ixKMHJ4n2rFWlw8WbvTm
tKFumm2z3z9JkijzJFj4sHETebaa2ip/TxeQvhFD/jEBB1XaqneDw1UaRll+6auA
K6naZ0LzZx2cOzJpn4xN
=TVTZ
-----END PGP SIGNATURE-----
gpg: Signature made Fri 23 Sep 2016 02:58:53 PM PDT
gpg:                using RSA key 0x26801BB6012EA5D9
gpg: BAD signature from "Will Sargent <will.sargent@lightbend.com>" [ultimate]
```

but using the signing key does work:

```
 ✘  ~   echo "$(uname -a)" | gpg --armor --clearsign --default-key  0x26801BB6012EA5D9
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
-----BEGIN PGP SIGNATURE-----
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=jQY+
-----END PGP SIGNATURE-----
 ~  gpg
gpg: Go ahead and type your message ...
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

Linux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
-----BEGIN PGP SIGNATURE-----

iQIcBAEBCgAGBQJX5aagAAoJECaAG7YBLqXZvZwP/21yoEQ3hI9hP0QyrFJu/T/0
sD9Y+DGQHBU5WaII3/iVgBN2V3EQwlzX8dP4/LfTm7NQ0M2flgbPbqf/rUYLQQZg
lO489XbI78kk80b3kDebkautN5rQhkU0ZAy/WfDdKmwGnF1hEXzYqEwI5S0tGddT
cKt0U3cZ0XuOI7pdtSOD423tNV4l45sIAT/ndAsgpbzT0ZTkza65V/RHWqGQDDT1
VB6WKmuqOca1gTDYGlW5yITfOqdWjB30ljLjuOjFJjcOunJLinux Puget-153699 4.4.0-38-generic #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
QmlSRDMGyjfdzF3ec
X1/+vLKnI0M2ipFaxKTtjdCTo8+26wjExdGca6Sy8v9M0zBjA2vgCGBTwCpXkMQE
4HFZ6N0+6k/3icyNALJhHSRkApNom3ZqINntDNNcN/tyHZVUijb5/hfv7W4D5LSe
8b1/UbF/R46w21sgR4Rzfv5EsbZkkjWx65hTXYWByf4PqZ7NiJJGbETpPC8wSc+4
oZNk9SLZunzE2Gemk2CXu7VXR58BIP014FHjU4FN7k54ZGn7IzU2xfKCZ+se7pFh
SzWIrDhZP5vsbCMbh4HzD4WFPLteNOdV+nkHi4iaSXc7UQfdgZIeKb2ljbjJTmN4
fyi/Zjk0+29pwB+W5iWD4AoKqzSsHMCrK73KRyAHcFaHOHILl8grG0GsfJmPGHCz
Mm3O7IH5is7ZkvOmbUMY
=jQY+
-----END PGP SIGNATURE-----

gpg: Signature made Fri 23 Sep 2016 03:03:12 PM PDT
gpg:                using RSA key 0x26801BB6012EA5D9
gpg: Good signature from "Will Sargent <will.sargent@lightbend.com>" [ultimate]
gpg:                 aka "Will Sargent <will.sargent@gmail.com>" [ultimate]
Primary key fingerprint: 75E4 E7F9 1D18 D981 3028  64B1 B1A9 D5A2 A605 F794
     Subkey fingerprint: ADB3 1ED0 EC01 44AF 8301  320D 2680 1BB6 012E A5D9
```

README.md

@@ -1121,9 +1121,9 @@ Decrypt the sample text by pasting it:
 
 ### Signing
 
-Sign some sample text:
+Sign some sample text using the signing subkey:
 
-    $ echo "$(uname -a)" | gpg --armor --clearsign --default-key 0xFF3E7D88647EBCDB
+    $ echo "$(uname -a)" | gpg --armor --clearsign --default-key 0xBECFA3C1AE191D15
     -----BEGIN PGP SIGNED MESSAGE-----
     Hash: SHA512