Add documentation and documentation testing

docs/src/prepareTrustline.md.ejs

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+## prepareTrustline
+
+`prepareTrustline(address: string, trustline: Object, instructions: Object): Promise<Object>`
+
+Prepare a trustline transaction. The prepared transaction must subsequently be [signed](#sign) and [submitted](#submit).
+
+### Parameters
+
+<%- renderSchema("input/prepare-trustline.json") %>
+
+### Return Value
+
+This method returns a promise that resolves with an object with the following structure:
+
+<aside class="notice">
+All "prepare*" methods have the same return type.
+</aside>
+
+<%- renderSchema("output/prepare.json") %>
+
+### Example
+
+```javascript
+const address = 'r9cZA1mLK5R5Am25ArfXFmqgNwjZgnfk59';
+const trustline = <%- importFile('test/fixtures/requests/prepare-trustline.json') %>;
+return api.preparePayment(address, trustline).then(prepared =>
+  {/* ... */});
+```
+
+<%- renderFixture("responses/prepare-trustline.json") %>