Adding links to README features section

[axsaucedo]

Added links to main features, and added line that links to security policy

[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[seldondev]

Fri Aug 14 06:47:48 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2278/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2278 --build=2

[seldondev]

Fri Aug 14 06:47:48 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2278/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2278 --build=1

[seldondev]

@axsaucedo: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pr-build	2aeb9e0	link	/test this

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