IC:2021-03-23, Add cFE PRs

nasa/cFE#1218 - Adds a local definition of `SOFTWARE_BIG/LITTLE_BIT_ORDER` directly inside `cfe_endian.h` to provide a compatible symbol for apps that still require this. This allows CFE to build and run successfully when OSAL stops providing this in `common_types.h`.
nasa/cFE#1193 - Removes incorrect statements from Application Developers Guide
nasa/cFE#1235 - Fixes truncation handling on vsnprintf error by adding a cast to avoid implicit conversion
nasa/cFE#1220 - Clarify the documentation on SB MsgId regarding requirements for command and telemetry messages
nasa/cFE#1230 - Avoids undefined behavior and resolves static analysis warnings by casting `isspace` input to `unsigned char`.
nasa/cFE#1231 - Updates message module and msgid v1, `CFE_MSG_SetMsgId`, to use mask instead of cast to alter value. Resolves static analysis warning.
nasa/cFE#1232 - Updates `CFE_ES_FileWriteByteCntErr` to report status, not a `size_t` actual since `OS_write` returns `int32`. Use `int16` for local type from `CFE_TBL_FindTableInRegistry` since it's an index, not a status.
nasa/cFE#1228 - Replaces `<>` with `"` in local `#include`s
nasa/cFE#1227 - Adds `CONTRIBUING.md` that links to the main cFS contributing guide.