reorganize limitations in README

Author: timgraham

README.md

@@ -82,19 +82,35 @@ DATABASES = {
 
 ## Limitations
 
-Limitation|Comment|Resolution
----|---|---
-Lack of DEFAULT for columns|Cloud Spanner doesn't support using DEFAULT for columns thus the use of default values might have to enforced in your controller logic|
-Lack of FOREIGN KEY constraints|Cloud Spanner doesn't support foreign key constraints thus they have to be defined in code
-Lack of sequential and auto-assigned IDs|Cloud Spanner doesn't autogenerate IDs and this integration instead creates UUID4 to avoid [hotspotting](https://cloud.google.com/spanner/docs/schema-design#uuid_primary_key) so you SHOULD NOT rely on IDs being sorted|We generate UUID4s for each AutoField
-Numeric values are saved as FLOAT64|Cloud Spanner doesn't support the NUMERIC type thus we might have precision losses|Decimal and Numeric are translated to FLOAT64. If Cloud Spanner adds NUMERIC in the future, you might need to migrate your columns
-Optimistic transactions when running DDL and other statements|Cloud Spanner CANNOT run DDL (CREATE, DROP, ALTER) in a Transaction and CAN ONLY run them in the [DatabaseAdmin RPC]() so any mixes of DDL and other statements will make prior statements get run, DDL run, then following statements run separately|
-107 unexplored Django tests|We need to run every single test in the Django test suite for 100% certainty|Currently in progress to complete the [211 test cases](https://gist.github.com/odeke-em/d3476b6cb7c3485de6aa29956ed50a75#file-all_tests-txt)
-Table names CANNOT have spaces within them whether back-ticked or not|Cloud Spanner DOEST NOT support tables with spaces in them for example `Entity ID`|Ensure that your table names don't have spaces within them
-Table names CANNOT have punctuation marks and MUST contain valid UTF-8|Cloud Spanner DOEST NOT support punctuation marks e.g. periods ".", question marks "?" in table names|Ensure that your table names don't have punctuation marks
-Lack of stored procedures and triggers|Cloud Spanner DOES NOT support stored procedures nor triggers so if your code relies on them, you might need to move the logic into your application|Not supported
-Lack of VARIANCE, STDDEV database functions|Cloud Spanned doesn't yet implement these functions|
+### `AutoField` generates random IDs
 
+Spanner doesn't have a way to auto-generate primary key values. Instead,
+django-spanner monkeypatches `AutoField` to generate a random UUID4. It
+generates a default using `Field`'s `default` option which means `AutoField`s
+will have a value when a model instance is created. For example:
+
+```
+>>> ExampleModel()
+>>> ExampleModel.pk
+4229421414948291880
+```
+
+To avoid [hotspotting](https://cloud.google.com/spanner/docs/schema-design#uuid_primary_key),
+these IDs are not monotonically increasing. This means that sorting models by
+id isn't guaranteed to return them in the order in which they were created.
+
+### `ForeignKey` constraints aren't created
+
+django-spanner [doesn't create foreign key constraints](https://github.com/googleapis/python-spanner-django/issues/313).
+
+### `DecimalField` isn't supported
+
+Spanner doesn't support a NUMERIC data type that allows storing high precision
+decimal values without the possibility of data loss.
+
+### `Variance` and `StdDev` database functions aren't supported
+
+Spanner doesn't have these functions.
 
 ## How it works