Rewords the rules pitfall document

So it stop suggesting that it is hard to write rules, though it is not easy to write good rules :)

--
MOS_MIGRATED_REVID=129423968
diff --git a/site/_layouts/documentation.html b/site/_layouts/documentation.html
index 3a8252b..ec17b6a 100644
--- a/site/_layouts/documentation.html
+++ b/site/_layouts/documentation.html
@@ -74,7 +74,7 @@
               <li><a href="/docs/skylark/rules.html">Rules</a></li>
               <li><a href="/docs/skylark/aspects.html">Aspects</a></li>
               <li><a href="/docs/skylark/repository_rules.html">Repository rules</a></li>
-              <li><a href="/docs/hard_to_write_rules.html">Pitfalls</a></li>
+              <li><a href="/docs/rule-challenges.html">Challenges of writing rules</a></li>
               <li>
                 <a class="sidebar-nav-heading" data-toggle="collapse"
                     href="#skylark-lib-menu" aria-expanded="false"
diff --git a/site/contributing.md b/site/contributing.md
index 6ea17d4..ab60ff2 100644
--- a/site/contributing.md
+++ b/site/contributing.md
@@ -139,8 +139,8 @@
     you want to add rules, consider using [Skylark](docs/skylark/index.html)
     first.
   * Builtin rules in `com.google.devtools.build.lib.rules` and in
-    `com.google.devtools.build.lib.bazel.rules`. You might want to read [Why is
-    it so difficult to write Bazel rules?](hard_to_write_rules.html) first.
+    `com.google.devtools.build.lib.bazel.rules`. You might want to read about
+    the [Challenges of Writing Rules](rule-challenges.html) first.
 * Java native interfaces in `src/main/native`.
 * Various tooling for language support (see the list in the
   [compiling Bazel](#compile-bazel) section).
diff --git a/site/docs/hard_to_write_rules.md b/site/docs/rule-challenges.md
similarity index 95%
rename from site/docs/hard_to_write_rules.md
rename to site/docs/rule-challenges.md
index e664abb..7b92655 100644
--- a/site/docs/hard_to_write_rules.md
+++ b/site/docs/rule-challenges.md
@@ -1,15 +1,15 @@
 ---
 layout: documentation
-title: Why is it so difficult to write Bazel rules?
+title: Challenges of Writing Rules.
 ---
 
-# Why is it difficult to write Bazel rules?
+# Challenges of Writing Rules.
 
-We have heard feedback from various people multiple times that Bazel rules are
-hard to write. There is no single root cause, but it’s due to a combination of
-historical circumstances and intrinsic complexity in the problem domain. This
-document attempts to give a high level overview of the specific issues that we
-believe to be the main contributors.
+We have heard feedback from various people that they have
+difficulty to write efficient Bazel rules. There is no single root cause, but
+it’s due to a combination of historical circumstances and intrinsic complexity
+in the problem domain. This document attempts to give a high level overview of
+the specific issues that we believe to be the main contributors.
 
 * Assumption: Aim for Correctness, Throughput, Ease of Use & Latency
 * Assumption: Large Scale Repositories