GODRIVER-2287 Add section to troubleshooting FAQ per driver with top …

…SEO results (mongodb#1017)

Co-authored-by: Benjamin Rewis <32186188+benjirewis@users.noreply.github.com>

Co-authored-by: Benjamin Rewis <32186188+benjirewis@users.noreply.github.com>

Co-authored-by: Benjamin Rewis <32186188+benjirewis@users.noreply.github.com>

Co-authored-by: Benjamin Rewis <32186188+benjirewis@users.noreply.github.com>

README.md

@@ -227,13 +227,18 @@ Ensure the [`--networkMessageCompressors` flag](https://www.mongodb.com/docs/man
 -------------------------
 ## Contribution
 
-Check out the [project page](https://jira.mongodb.org/browse/GODRIVER) for tickets that need completing. See our [contribution guidelines](CONTRIBUTING.md) for details.
+Check out the [project page](https://jira.mongodb.org/browse/GODRIVER) for tickets that need completing. See our [contribution guidelines](docs/CONTRIBUTING.md) for details.
 
 -------------------------
 ## Continuous Integration
 
 Commits to master are run automatically on [evergreen](https://evergreen.mongodb.com/waterfall/mongo-go-driver).
 
+-------------------------
+## Frequently Encountered Issues
+
+See our [common issues](docs/common-issues.md) documentation for troubleshooting frequently encountered issues.
+
 -------------------------
 ## Thanks and Acknowledgement
 

docs/common-issues.md

@@ -0,0 +1,52 @@
+# Frequently Encountered Issues
+
+## `WriteXXX` can only write while positioned on a Element or Value but is positioned on a TopLevel
+
+The [`bson.Marshal`](https://pkg.go.dev/go.mongodb.org/mongo-driver/bson#Marshal) function requires a parameter that can be decoded into a BSON Document, i.e. a [`primitive.D`](https://github.com/mongodb/mongo-go-driver/blob/master/bson/bson.go#L31). Therefore the error message
+
+> `WriteXXX` can only write while positioned on a Element or Value but is positioned on a TopLevel
+
+occurs when the input to `bson.Marshal` is something *other* than a BSON Document. Examples of this occurrence include
+
+- `WriteString`: the input into `bson.Marshal` is a string
+- `WriteNull`: the input into `bson.Marshal` is null
+- `WriteInt32`: the input into `bson.Marshal` is an integer
+
+Many CRUD operations in the Go Driver use `bson.Marshal` under the hood, so it's possible to encounter this particular error without directly attempting to encode data. For example, when using a sort on [`FindOneAndUpdate`](https://pkg.go.dev/go.mongodb.org/mongo-driver/mongo#Collection.FindOneAndUpdate) this error can occur when not properly initializing the `sort` variable:
+
+```go
+var sort bson.D // this is nil and will result in a WriteNull error
+opts := options.FindOneAndUpdate().SetSort(sort)
+update := bson.D{{"$inc", bson.D{{"x", 1}}}}
+sr := coll.FindOneAndUpdate(ctx, bson.D{}, update)
+if err := sr.Err(); err != nil {
+	log.Fatalf("error getting single result: %v", err)
+}
+```
+
+The above example is resolved by initializing the `sort` variable:
+
+```go
+sort := bson.D{}
+```
+
+## Convert BSON Document to JSON
+
+There are a variety of marshalers that can be used to encode a BSON document as JSON, including [MarshalExtJSON](https://pkg.go.dev/github.com/mongodb/mongo-go-driver/bson#MarshalExtJSON):
+
+```go
+doc := bson.D{{"x", 1}}
+
+jsonBytes, err := bson.MarshalExtJSON(doc, true, false)
+if err != nil {
+	log.Fatalf("error encoding json: %v", err)
+}
+
+m := make(map[string]interface{})
+if err := json.Unmarshal(jsonBytes, &m); err != nil {
+	log.Fatalf("error decoding json: %v", err)
+}
+fmt.Printf("json: %v\n", m)
+```
+
+