Add warning about lost custom operations

[dchristensen]

Warning about losing custom operations when resetting/squashing migrations.

[smitpatel]

Does it need to be added to the docs though? If you are deleting "Migrations" folder then you are going to lose all data inside it, custom or not-custom. Before deleting any source code file, you should evaluate if it contains anything you want to save.

[dchristensen]

My thinking was that although you are deleting the "Migrations" folder you are expecting EF to re-generate that code (just squashed into a single file). However, if you have added any customizations those will not be re-generated and will be lost. The resulting "squashed" migration will not contain those original customizations and will therefore be different from the expected output.

[smitpatel]

@dchristensen - Thank you.