Document pre-compiled schemas for CSP in README

[GrahamLea]

Pre-compiled schemas are a workaround for maintaining a secure Content Security Policy (CSP)

What issue does this pull request resolve?
#1228

What changes did you make?
Added two paragraphs to the README

Is there anything that requires more attention while reviewing?
Unlikely

[GrahamLea]

I've added this under the "Using in browser" section, but it could also fit in the "Security considerations" section.
I didn't add it to the menu at the top as I wasn't sure whether it needed to contain all level 3 headings.

[epoberezkin]

Thank you. I think it is important to comment somehow that the schemas, if they successfully compile, are equivalent, but that not everything that can be compile with ajv can be processed with ajv-pack. As is the change may create impression that the compiled schemas are not equivalent.

Re placement - I don't mind either way, it's ok in browser section, there can be a link from security section if you like.

[GrahamLea]

I think it is important to comment somehow that the schemas, if they successfully compile, are equivalent, but that not everything that can be compile with ajv can be processed with ajv-pack.

Ah, yeah, thanks. I wasn't totally sure on what the deal was there. i.e. whether the limitations were in compiling or in the functionality of the compiled schemas. I've revised that text now to be clearer.

Also added a Contents link and a link from the Security section.