DOCS: Improve docs/writing-providers.md wrt registrars and testing (#1366)

* DOCS: Improve docs/writing-providers.md wrt registars and testing

docs/writing-providers.md

@@ -98,8 +98,9 @@ into three general categories:
 has A and MX records), you have to replace all the records at that
 label. (GANDI_V5)
 * **incremental-label-type:** Like incremental-record, but updates to any records at a label have to be done by type.  For example, if a label (www.example.com) has many A and MX records, even the smallest change to one of the A records requires replacing all the A records. Any changes to the MX records requires replacing all the MX records.  If an A record is converted to a CNAME, one must remove all the A records in one call, and add the CNAME record with another call.  This is deceptively difficult to get right; if you have the choice between incremental-label-type and incremental-label, pick incremental-label. (DESEC, ROUTE53)
+* **registrar only:** These providers are registrars but do not provide DNS service. (CSCGLOBAL, EASYNAME, INTERNETBS, OPENSRS)
 
-All providers use the "diff" module to detect differences. It takes
+All DNS providers use the "diff" module to detect differences. It takes
 two zones and returns records that are unchanged, created, deleted,
 and modified.
 The zone providers use the
@@ -128,10 +129,21 @@ Add the provider list so DNSControl knows it exists.
 
 ## Step 5: Implement
 
-Implement all the calls in
-[providers.DNSServiceProvider interface.](https://godoc.org/github.com/StackExchange/dnscontrol/providers#DNSServiceProvider).
+**If you are implementing a DNS Service Provider:**
 
-The function `GetDomainCorrections` is a bit interesting. It returns
+Implement all the calls in the
+[providers.DNSServiceProvider interface.](https://pkg.go.dev/github.com/StackExchange/dnscontrol/providers#DNSServiceProvider).
+
+The function `GetDomainCorrections()` is a bit interesting. It returns
+a list of corrections to be made. These are in the form of functions
+that DNSControl can call to actually make the corrections.
+
+**If you are implementing a DNS Registrar:**
+
+Implement all the calls in the
+[providers.Registrar interface.](https://pkg.go.dev/github.com/StackExchange/dnscontrol/providers#Registrar).
+
+The function `GetRegistrarCorrections()` returns
 a list of corrections to be made. These are in the form of functions
 that DNSControl can call to actually make the corrections.
 
@@ -171,6 +183,22 @@ export R53_KEY='CHANGE_TO_THE_KEY'
 go test -v -verbose -provider ROUTE53
 ```
 
+Some useful `go test` flags:
+
+* Slow tests? Add `-timeout n` to increase the timeout for tests
+  * `go test` kills the tests after 10 minutes by default.  Some providers need more time.
+  * This flag must be *before* the `-verbose` flag.  Usually it is the first flag after `go test`.
+  * Example:  `go test -timeout 20m -v -verbose -provider CLOUDFLAREAPI`
+* Run only certain tests using the `-start` and `-end` flags.
+  * Rather than running all the tests, run just the tests you want.
+  * These flags must be *after* the `-provider FOO` flag.
+  * Example: `go test -v -verbose -provider ROUTE53 -start 10 -end 20` run tests 10-20 inclusive.
+  * Example: `go test -v -verbose -provider ROUTE53 -start 5 -end 5` runs only test 5.
+  * Example: `go test -v -verbose -provider ROUTE53 -start 20` skip the first 19 tests.
+  * Example: `go test -v -verbose -provider ROUTE53 -end 20` only run the first 20 tests.
+* If a test will always fail because the provider doesn't support the feature, you can opt out of the test.  Look at `func makeTests()` in [integrationTest/integration_test.go](https://github.com/StackExchange/dnscontrol/blob/2f65533e1b92c2967229a92a304fff7c14f7f4b6/integrationTest/integration_test.go#L675) for more details.
+
+
 ## Step 8: Update docs
 
 * Edit [README.md](https://github.com/StackExchange/dnscontrol): Add the provider to the bullet list.