Migration guide from apollo to Rover

[JakeDawkins]

we need solutions for people moving from apollo to Rover in all kinds of use cases

See #425

[JakeDawkins]

multiple schema files

➜  events git:(master) ✗ ll
total 28K
-rw-r--r-- 1 jake jake 433 Feb 18 23:04 company.graphql
-rw-r--r-- 1 jake jake 320 Feb 18 22:59 companyRole.graphql
-rw-r--r-- 1 jake jake 579 Feb 18 23:00 event.graphql
-rw-r--r-- 1 jake jake 166 Feb 18 23:00 index.graphql
-rw-r--r-- 1 jake jake  93 Feb 18 23:00 interfaces.graphql
-rw-r--r-- 1 jake jake 408 Feb 18 23:00 ticket.graphql
-rw-r--r-- 1 jake jake 334 Feb 18 23:01 user.graphql
➜  events git:(master) ✗ cat *.graphql | rover graph check concat-demo-xh74k --schema -
  INFO Validated the proposed subgraph against metrics from concat-demo-xh74k@current
  INFO Compared 1 schema changes against 0 operations
+--------+-------------+---------------------------------------+
| Change | Code        | Description                           |
+--------+-------------+---------------------------------------+
| PASS   | FIELD_ADDED | type `Company`: field `testAdd` added |
+--------+-------------+---------------------------------------+

  INFO View full details here
  INFO https://studio-staging.apollographql.com/service/concat-demo-xh74k/checks/c28b808f-5082-4f9f-bda3-a49d750d5159?schemaTag=current
➜  events git:(master) ✗

typeDefs split up into multiple source files:

• move the typedefs into their own .graphql files and cat them together before running commands