Add Common Pain Points Section

[jaredcnance]

Should include use cases that are not fully supported out-of-the-box but have workarounds that can be presented until an official solution is available:

• EF many-to-many support JsonApiDotNetCore#151

• DbUpdateConcurrencyException when creating resources with relationships with ConcurrencyStamp JsonApiDotNetCore#350

• Allow filtering on properties of HasMany relationships JsonApiDotNetCore#238

• ...