Added PreciousBlock rpc command

_includes/devdoc/bitcoin-core/rpcs/rpcs/preciousblock.md

@@ -0,0 +1,47 @@
+{% comment %}
+This file is licensed under the MIT License (MIT) available on
+http://opensource.org/licenses/MIT.
+{% endcomment %}
+{% assign filename="_includes/devdoc/bitcoin-core/rpcs/rpcs/preciousblock.md" %}
+
+##### PreciousBlock
+{% include helpers/subhead-links.md %}
+
+{% assign summary_preciousBlock="treats a block as if it were received before others with the same work." %}
+
+{% autocrossref %}
+
+*Added in Bitcoin Core 0.14.0*
+
+The `preciousblock` RPC {{summary_preciousBlock}} A later `preciousblock` call can override the effect of an earlier one. The effects of `preciousblock` are not retained across restarts.
+
+*Parameter #1---the block hash*
+
+{% itemplate ntpd1 %}
+- n: "Header Hash"
+  t: "string (hex)"
+  p: "Required<br>(exactly 1)"
+  d: "The hash of the block to mark as precious"
+
+{% enditemplate %}
+
+*Result---`null` plus error on failed remove*
+
+{% itemplate ntpd1 %}
+- n: "`result`"
+  t: "null"
+  p: "Required<br>(exactly 1)"
+  d: "JSON `null`.  The JSON-RPC error field will be set only if you entered an invalid block hash"
+
+{% enditemplate %}
+
+*Examples from Bitcoin Core 0.14.1*
+
+{% highlight bash %}
+bitcoin-cli preciousblock 000000000000000001517a0bac70b8cd6f27ee\
+1b50a8f12bf606ea6fb6d561cd
+{% endhighlight %}
+
+Result (no output from `bitcoin-cli` because result is set to `null`).
+
+{% endautocrossref %}