Added document for troubleshooting errors

TROUBLESHOOTING.md

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+# Troubleshooting dbt Constraints
+
+As it executes, dbt Constraints will log a number of messages when it cannot create a constraint. 
+A list of the messages is provided below with additional details on how to address the message.
+
+## Messages that are displayed on the front-end of the command line interface and in the dbt log
+
+```
+Skipping primary/unique key because a physical column name was not found on the table: {TABLE NAME} {TABLE COLUMNS}
+Skipping foreign key because a physical column was not found on the pk table: {PK TABLE NAME} {PK TABLE COLUMNS}
+Skipping foreign key because a physical column was not found on the fk table: {FK TABLE NAME} {FK TABLE COLUMNS}
+```
+- These error messages typically occur when a column is misspelled or if the test uses an expression instead of a column name. 
+- One solution can be adding the expression as an additional column in your model so that you can reference it in your constraint.
+
+
+```
+"Skipping foreign key because a we couldn't find the child table: model={FK TABLE NAME} or source={PK TABLE NAME}
+```
+- This is only expected to occur when a foreign key constraint is made with a source and dbt Constraints can't parse the reference to a source table.
+- The package is looking for something that looks like: `source("source_name", "table_name")` or `source('source_name', 'table_name')`
+- You may need to replace any dynamic variables with strings for the source name or table name.
+
+
+```
+Skipping {CONSTRAINT NAME} because of insufficient privileges: {FK TABLE NAME} referencing {PK TABLE NAME}
+Skipping {CONSTRAINT NAME} because of insufficient privileges: {TABLE NAME}
+```
+- You must have OWNERSHIP on the child FK table and you must have REFERENCES on the parent PK table.
+- For primary keys and unique keys, you need ownership on the table.
+- These errors most frequently apply to sources.
+- This can also indicate that one of your tables is actually a view
+
+
+```
+Skipping {CONSTRAINT NAME} because a PK/UK was not found on the PK table: {PK TABLE NAME} {PK TABLE COLUMNS}
+```
+- You either need to manually create a primary key/unique key or you need to add a test to the parent table and allow the package to create the constraint.
+- The package creates constraints in the order of primary keys, unique keys, foreign keys to allow parent constraints to be referenced by foreign keys.
+
+
+## Messages that are only displayed in the dbt log:
+
+```
+Skipping {CONSTRAINT NAME} because PK/UK already exists: {TABLE NAME} {TABLE COLUMNS}
+Skipping {CONSTRAINT NAME} because FK already exists: {FK TABLE NAME} {FK TABLE COLUMNS}
+```
+- Indicates duplicate constraints or that a constraint was already added to an incremental / snapshot table on a previous run
+- Typically not an issue