Add note to readme about CAP_SYS_PTRACE - See pi-hole#734 (pi-hole#966)

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

README.md

@@ -273,6 +273,7 @@ DNSMasq / [FTLDNS](https://docs.pi-hole.net/ftldns/in-depth/#linux-capabilities)
 - `CAP_NET_ADMIN`: modify routing tables and other network-related operations (in particular inserting an entry in the neighbor table to answer DHCP requests using unicast packets)
 - `CAP_SYS_NICE`: FTL sets itself as an important process to get some more processing time if the latter is running low
 - `CAP_CHOWN`: we need to be able to change ownership of log files and databases in case FTL is started as a different user than `pihole`
+- `CAP_SYS_PTRACE`: May be needed if Pi-hole status is reported incorrectly (see [#734](https://github.com/pi-hole/docker-pi-hole/issues/734))
 
 This image automatically grants those capabilities, if available, to the FTLDNS process, even when run as non-root.\
 By default, docker does not include the `NET_ADMIN` capability for non-privileged containers, and it is recommended to explicitly add it to the container using `--cap-add=NET_ADMIN`.\