Update the README with current requirements, and fix style

The README was still referencing gp-xerces even though it's no
longer required for ORCA, so this was removed. On top of that,
some Markdown formatting fixes, minor wordsmithing, typo fixes
as well as some whitespace cleanup and turned URLs into links.

README.md

@@ -44,23 +44,8 @@ Follow [these linux steps](README.linux.md) for getting your system ready for GP
 Currently GPDB assumes ORCA libraries and headers are available in the targeted
 system and tries to build with ORCA by default.  For your convenience, here are
 the steps of how to build the optimizer. For the most up-to-date way of
-building, see the README at the following repositories:
-
-* https://github.com/greenplum-db/gp-xerces
-* https://github.com/greenplum-db/gporca
-
-1. Install our patched version of Xerces-C
-
-    ```
-    git clone https://github.com/greenplum-db/gp-xerces
-    mkdir gp-xerces/build
-    cd gp-xerces/build
-    ../configure
-    make install
-    cd ../..
-    ```
-
-1. Install ORCA, the query optimizer:
+building, see the README in the
+[ORCA repository](https://github.com/greenplum-db/gporca).
 
     ```
     git clone https://github.com/greenplum-db/gporca
@@ -77,10 +62,10 @@ building, see the README at the following repositories:
     ```
 #### Using conan dependency manager
 
-    1. cd gpdb/depends
-    2. conan remote add conan-gpdb https://api.bintray.com/conan/greenplum-db/gpdb-oss
-    3. conan install --build
-       * This command will fetch the orca and xerces artifacts from bintray repository, build and install them.
+1. cd gpdb/depends
+2. conan remote add conan-gpdb https://api.bintray.com/conan/greenplum-db/gpdb-oss
+3. conan install --build
+   * This command will fetch the ORCA and xerces artifacts from bintray repository, build and install them.
    * The header and library files will be copied to the location specified by imports section of conanfile.txt in depends directory.
      In case, the files should be copied elsewhere, please change the location.
 
@@ -170,6 +155,7 @@ make installcheck-world
 ## Alternative Configurations
 
 ### Building GPDB without GPORCA
+
 Currently, GPDB is built with ORCA by default so latest ORCA libraries and headers need
 to be available in the environment. [Build and Install](#buildOrca) the latest ORCA.
 
@@ -183,11 +169,12 @@ make distclean
 ```
 
 ### Building GPDB with PXF
+
 PXF is an extension framework for GPDB to enable fast access to external hadoop datasets.
 Refer to [PXF extension](https://github.com/greenplum-db/gpdb/tree/master/gpAux/extensions/pxf) for more information.
 Currently, GPDPB isn't built with PXF by default.
 PXF requires curl version >= 7.21.3 and also has an additional dependancy on json-c library
-In order to build GPDB with pxf, simply change `configure` with additional option `--enable-pxf`.
+In order to build GPDB with pxf, simply invoke `./configure` with the additional option `--enable-pxf`.
 ```
 # Configure build environment to build/install PXF at /usr/local/gpdb
 ./configure --with-perl --with-python --with-libxml --prefix=/usr/local/gpdb --enable-pxf
@@ -234,7 +221,7 @@ Known issues:
 
 ### Running regression tests with Docker
 
-1. Create a docker host with 8gb RAM and 4 cores
+1. Create a docker host with 8GB RAM and 4 cores
     ```bash
     docker-machine create -d virtualbox --virtualbox-cpu-count 4 --virtualbox-disk-size 50000 --virtualbox-memory 8192 gpdb
     eval $(docker-machine env gpdb)
@@ -264,7 +251,7 @@ Known issues:
 
 ### Caveats
 
-* No Space Left On Device
+* No Space Left On Device:
     On macOS the docker-machine vm can periodically become full with unused images.
     You can clear these images with a combination of docker commands.
     ```bash
@@ -308,7 +295,7 @@ throughout the codebase, but a few larger additions worth noting:
 
 * __gpdb-doc/__
 
-  Constains the Greenplum documentation in DITA XML format. Refer to
+  Contains the Greenplum documentation in DITA XML format. Refer to
   `gpdb-doc/README.md` for information on how to build, and work with
   the documentation.
 
@@ -510,8 +497,8 @@ just commit to the repository directly.
 
 ## Documentation
 
-For Greenplum Database documentation, please check the online docs:
-http://greenplum.org/docs/
+For Greenplum Database documentation, please check the
+[online documentation](http://greenplum.org/docs/).
 
 For further information beyond the scope of this README, please see
 [our wiki](https://github.com/greenplum-db/gpdb/wiki)