Add documentation for disabling YARA rules

[austinbyers]

to: @ryandeivert
cc: @airbnb/binaryalert-maintainers
size: small

Background

Issue #95 brought some questions about fixing or removing YARA rules which aren't supported (e.g. with those with special modules).

Changes

• Adds a note to the documentation that we are working on a more expressive rule cloning
• Adds a section to the documentation on how to disable rules files or individual YARA rules

Testing

• sphinx-build -W docs/source docs/build and viewing built file locally

[coveralls]

Coverage remained the same at 92.787% when pulling d2e2ac2 on austin-disable-docs into b319e9f on master.

[coveralls]

Coverage remained the same at 92.787% when pulling d2e2ac2 on austin-disable-docs into b319e9f on master.

[ryandeivert]

LGTM!