[Tile-serving] [openstreetmap/osm2pgsql] Added libgeos dependencies to README (#714)

[jnachtigall]

Update the docs. Without the libgeos packages I had the problem:

```
# cmake ..
...
CMake Warning at cmake/FindOsmium.cmake:163 (message):
  Osmium: GEOS library is required but not found, please install it or
  configure the paths.
Call Stack (most recent call first):
  CMakeLists.txt:95 (find_package)


CMake Error at /usr/share/cmake-3.0/Modules/FindPackageHandleStandardArgs.cmake:136 (message):
  Could NOT find Osmium (missing: GEOS_INCLUDE_DIR GEOS_LIBRARY)
Call Stack (most recent call first):
  /usr/share/cmake-3.0/Modules/FindPackageHandleStandardArgs.cmake:343 (_FPHSA_FAILURE_MESSAGE)
  cmake/FindOsmium.cmake:262 (find_package_handle_standard_args)
  CMakeLists.txt:95 (find_package)


-- Configuring incomplete, errors occurred!
```
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/osm2pgsql/pull/714

-- Commit Summary --

  * Added libgeos dependencies to README

-- File Changes --

    M README.md (2)

-- Patch Links --

https://github.com/openstreetmap/osm2pgsql/pull/714.patch
https://github.com/openstreetmap/osm2pgsql/pull/714.diff

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