Add status badges to the README

[ac000]

This adds a 'repostatus' badge, set to 'active', as per F5/NGINX OSS
policy...

It also adds a GitHub workflows badge that shows the current state of our
CI builds and points to the Unit workflows page.

You can see what they look like here.

Note: For the workflows badge (the second one) the 'CI' text (on the
black background) can be changed.

[tippexs]

LGTM

[ac000]

Rebase with master

$ git range-diff db7d2670...7472a2ca
 -:  -------- >  1:  6b138571 Wasm-wc: Bump the mio crate from 0.8.10 to 0.8.11
 -:  -------- >  2:  2e615250 Add dependabot.yml
 -:  -------- >  3:  0d99744d Router: match when pattern and tested string are both zero length
 -:  -------- >  4:  fdc46759 NJS: avoiding arithmetic ops with NULL pointer in r->args
 -:  -------- >  5:  8844d33c Fixed undefined behaviour in left shift of int value
 -:  -------- >  6:  7dcd6c0e Avoiding arithmetic ops with NULL pointer in nxt_http_arguments_parse
 -:  -------- >  7:  264b3755 Avoiding arithmetic ops with NULL pointer in nxt_port_mmap_get
 -:  -------- >  8:  c9461a6b Initialize port_impl only when it is needed
 -:  -------- >  9:  dd701fb4 Avoiding arithmetic ops with NULL pointer in nxt_unit_mmap_get
 -:  -------- > 10:  abcfc4cd Fix the security-alert email link in the README
 -:  -------- > 11:  9993814d NJS: loader should be registered using njs_vm_set_module_loader()
 -:  -------- > 12:  0716b0c7 Tests: NJS cacheable variables with access log
 1:  8b4f0129 = 13:  dc16a7bc Add a repostatus badge to the README
 2:  db7d2670 = 14:  7472a2ca Add a GitHub workflow status badge for our CI to the README