Corrections, additions to initial doc (#600)

* Correct/clarify build/install instructions

GTest is google test, don't obsfucate needlessly for newcomers.
Adding google test into installation guide helps newcomers.
Third option under this  line: "Note that Google Benchmark requires Google Test to build and run the tests. This
dependency can be provided three ways:"
Was not true (did not occur). If there is a further option that needs to be specified in order for that functionality to work it needs to be specified.

* Add prerequisite knowledge section

A lot of assumptions are made about the reader in the documentation. This is unfortunate.

* Removal of abbreviations for google test
diff --git a/README.md b/README.md
index dbf59d7..b2e8eb6 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@
 
 ```bash
 $ git clone https://github.com/google/benchmark.git
-# Benchmark requires GTest as a dependency. Add the source tree as a subdirectory.
+# Benchmark requires Google Test as a dependency. Add the source tree as a subdirectory.
 $ git clone https://github.com/google/googletest.git benchmark/googletest
 $ mkdir build && cd build
 $ cmake -G <generator> [options] ../benchmark
@@ -31,15 +31,13 @@
 $ make
 ```
 
-Note that Google Benchmark requires GTest to build and run the tests. This
-dependency can be provided three ways:
+Note that Google Benchmark requires Google Test to build and run the tests. This
+dependency can be provided two ways:
 
-* Checkout the GTest sources into `benchmark/googletest`.
+* Checkout the Google Test sources into `benchmark/googletest` as above.
 * Otherwise, if `-DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON` is specified during
   configuration, the library will automatically download and build any required
   dependencies.
-* Otherwise, if nothing is done, CMake will use `find_package(GTest REQUIRED)`
-  to resolve the required GTest dependency.
 
 If you do not wish to build and run the tests, add `-DBENCHMARK_ENABLE_GTEST_TESTS=OFF`
 to `CMAKE_ARGS`.
@@ -61,6 +59,7 @@
 ```
 git clone https://github.com/google/benchmark.git
 cd benchmark
+git clone https://github.com/google/googletest.git
 mkdir build
 cd build
 cmake .. -DCMAKE_BUILD_TYPE=RELEASE
@@ -88,6 +87,11 @@
 this branch. However, this branch provides no stability guarantees and reserves
 the right to change and break the API at any time.
 
+##Prerequisite knowledge
+
+Before attempting to understand this framework one should ideally have some familiarity with the structure and format of the Google Test framework, upon which it is based. Documentation for Google Test, including a "Getting Started" (primer) guide, is available here:
+https://github.com/google/googletest/blob/master/googletest/docs/Documentation.md
+
 
 ## Example usage
 ### Basic usage