Add documentation about fuzzer execution environment.

README.md

@@ -106,6 +106,8 @@ oss-fuzz will adhere to following disclosure principles:
   *part of target* source code repository.
 * [Running and Building External Fuzzers](docs/building_running_fuzzers_external.md) documents the process for fuzzers that are
   *part of oss-fuzz* source code repository.
+* [Fuzzer execution environment](docs/fuzzer_environment.md) documents the
+  environment under which your fuzzers will be run.
 * [Targets List](targets/README.md) lists OSS targets added to oss-fuzz.
 * [Chrome's Efficient Fuzzer Guide](https://chromium.googlesource.com/chromium/src/testing/libfuzzer/+/HEAD/efficient_fuzzer.md) 
   while contains some chrome-specifics, is an excellent documentation on making your fuzzer better.

docs/fuzzer_environment.md

@@ -0,0 +1,20 @@
+# Fuzzer environment on ClusterFuzz
+
+Your fuzzers will be run on ClusterFuzz (Linux environment) with some
+restrictions.
+
+## Current working directory
+
+You cannot make any assumptions about the current working directory of your
+fuzzer. If you need to load data files, please use `argv[0]` to get the
+directory where your fuzzer executable is located. This may change in the near
+future.
+
+## Filesystem
+
+Everything except `/tmp` is read-only, including the directory that your fuzzer
+executable lives in. Note that `/tmp` is limited in size (64MB).
+
+## Network access
+
+There will be no network interfaces available (not even loopback).

docs/new_target.md

@@ -72,6 +72,12 @@ There are [lots](../targets/libxml2/libxml2_xml_read_memory_fuzzer.cc)
 [of](../targets/expat/parse_fuzzer.cc) [examples](../targets/zlib/zlib_uncompress_fuzzer.cc)
 in this project repository.
 
+### Fuzzer execution environment
+
+[This page](fuzzer_environment.md) gives information about the environment that
+your fuzzers will run under on ClusterFuzz, and the assumptions that you can
+make.
+
 ## build.sh
 
 This is where most of the work is done to build fuzzers for your target. The script will