Notes on README

@nelsonic This README is AWESOME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (I know this is not what issues are for but it really is amazing. Thanks so much)

I have gone through and made some points, and wanted to list them here. I can separate them out into different issues if needed and start working on them but just wanted to have them documented somewhere.

### Prerequisites?
> Basic understanding of Ecto (the module used to interface with databases in elixir/phoenix)

There is no link for this point (The previous points have links to learning repos). I have looked through the [learn-phoenix-framework](https://github.com/dwyl/learn-phoenix-framework) repo but couldn't find anything specific to just learning the basics of ecto (almost all the examples have something to do with ecto in them but nothing ecto specific). I was thinking that we could make a tutorial in the learn-phoenix repo that introduces that basics of ecto (migration/schema/repo) (I would be up for making this just to be clear 👍)

### Time Requirement?
> Simply reading ("skimming") through this example will only take 15 minutes.
Following the examples on your computer (to fully understand it) will take around 1 hour

Personally, I'm not a fan of the time thing as I feel it can make people feel down if they take longer. Just my opinion though.

## How

Before the
```elixir
mix ecto.migrate
```
command, we need to edit the migration file. It needs the following line added to it
```elixir
create(unique_index(:users, [:email_hash]))
```

This missing line was causing tests to fail later on in the example.

## 3. Define The 6 Functions

### 3.4 Hash Email Address

These tests depend on the `:secret_key_base`. They were failing for me following the example because mine was different. Not sure how best to resolve this. Could we use a pattern match similar to the [decrypt function](https://github.com/dwyl/phoenix-ecto-encryption-example/blob/master/lib/encryption/aes.ex#L66).

## 4 Create `EncryptedField` Custom Ecto Type

I know you suggest users write their own tests at this point so not really a problem but just thought I would mention this. The tests in [test/lib/encrypted_field_test.exs](https://github.com/dwyl/phoenix-ecto-encryption-example/blob/master/test/lib/encrypted_field_test.exs) do not test the `load/1` function (only the load/2). This function may not exist for people following the readme. (Again, not an issue, just mentioning it 👍)

## 5. Use EncryptedField Ecto Type in User Schema

```elixir
alias Encryption.EncryptedField
```

Also need to add `User` as an `alias` at this point as it is used in the `changeset` function.

## 8. Create PasswordField Ecto Type for Hashing Email Address
> def verify_pass(password, stored_hash) do
Argon2.verify_pass(password, stored_hash)
end

This functions name has been changed in error. Should be `verify_password`. Caused tests in `password_field_tests.exs` to fail.

### 10.2 Re-order :email_hash Field in User Schema

users schema never shows adding a `:password` field during the example
```elixir
field :password, :binary, virtual: true # virtual means "don't persist"`
```

This doesn't break any tests or anything. Just something I noticed. Was it added for the user interface mentioned in the conclusion?