doc(README): overhaul with TOC and filled in

[msimerson]

Structural improvements, fixes #65

• Added a Table of Contents with anchor links to every method
• Added a Quick Start section with the 3 most common use cases
• Each method now has its own heading

Content corrections

• The old README said "four global methods"; the actual count is seven, all are now documented
• Fixed broken examples: subnetMaskFromPrefixLength(24) == '255.255.255.0' and similar comparisons silently evaluate to false because the method returns an IPv4 object, not a string — corrected to use .toString()
• Updated source line anchors for the range tables (IPv4: 182→184, IPv6: 530→562)

New documentation

• parseCIDR, subnetMatch, fromByteArray (global)
• isIPv4, isIPv6 — lightweight format checks
• isValidFourPartDecimal, isValidCIDRFourPartDecimal — explained as strict-mode validators
• toFixedLengthString, toRFC5952String
• broadcastAddressFromCIDR / networkAddressFromCIDR — existed but without examples
• subnetMatch instance method on both IPv4 and IPv6
• prefixLengthFromSubnetMask for IPv6

View it rendered in GHFM here in my fork. Make comments and suggestions.

[whitequark]

Thanks, this looks very well-written and comprehensive! I admit I haven't had the time available to proofread the entire thing but I don't think it's really necessary here.

[msimerson]

Hey, in 9 more years I'll drop by and do it again. 😁