change some sentences to help understand

Pip install tempest will install a package from pip source, not from
a dir,so change it to pip install tempest/

Also change some sentences to help understand

Change-Id: I034b3e94fc35db40bd7f7f68ab20cdcbcfddbc37

README.rst

@@ -22,8 +22,8 @@ Tempest Design Principles that we strive to live by.
 - Tempest should not touch private or implementation specific
   interfaces. This means not directly going to the database, not
   directly hitting the hypervisors, not testing extensions not
-  included in the OpenStack base. If there is some feature of
-  OpenStack that is not verifiable through standard interfaces, this
+  included in the OpenStack base. If there are some features of
+  OpenStack that are not verifiable through standard interfaces, this
   should be considered a possible enhancement.
 - Tempest strives for complete coverage of the OpenStack API and
   common scenarios that demonstrate a working cloud.
@@ -47,10 +47,11 @@ working dir is the actually Tempest source repo, and there are a number of
 assumptions related to that. For this section we'll only cover the newer method
 as it is simpler, and quicker to work with.
 
-#. You first need to install Tempest this is done with pip, after you check out
-   the Tempest repo you simply run something like::
+#. You first need to install Tempest. This is done with pip after you check out
+   the Tempest repo::
 
-    $ pip install tempest
+    $ git clone https://github.com/openstack/tempest/
+    $ pip install tempest/
 
    This can be done within a venv, but the assumption for this guide is that
    the Tempest cli entry point will be in your shell's PATH.