chore(docs): add documentation regarding prismaClientImportId (#53)

Co-authored-by: Jason Kuhrt <jasonkuhrt@me.com>

README.md

@@ -598,10 +598,36 @@ query {
   })
 
   $settings({
-    prismaClientContextField = 'db', // <-- Tell Nexus Prisma
+    prismaClientContextField: 'db', // <-- Tell Nexus Prisma
   })
   ```
 
+##### `prismaClientImportId: string`
+
+- **@summary** Where Nexus Prisma will try to import your generated Prisma Client from. You should not need to configure this normally because Nexus Prisma generator automatically reads the Prisma Client generator `output` setting if you have set it. The value here will be used in a dynamic import thus following Node's path resolution rules. You can pass a node_modules package like `foo` `@prisma/client`, `@my/custom/thing` etc. or you can pass an absolute module/file path `/my/custom/thing` / `/my/custom/thing/index.js` or finally a relative path to be resolved relative to the location of Nexus Prisma source files (you probably don't want this).
+
+- **@remarks** Nexus Prisma imports Prisma client internally for two reasons: 1) validation wherein a class reference to Prisma Client is needed for some `instanceof` checks and 2) for acquiring the DMMF as Nexus Prisma relies on some post-processing done by Prisma Client generator.
+
+- **@example**
+
+  ```ts
+  // src/main.ts
+
+  import { PrismaClient } from '@prisma/client'
+  import { ApolloServer } from 'apollo-server'
+  import { makeSchema } from 'nexus'
+  import { User, Post, $settings } from 'nexus-prisma'
+
+  new ApolloServer({
+    schema: makeSchema({
+      types: [],
+    }),
+  })
+
+  $settings({
+    prismaClientImportId: '@my/custom/thing',
+  })
+
 ### Generator Settings
 
 You are able to control certain aspects of the Nexus Prisma code generation.